Plot legend customization

Thursday, July 12th, 2018

Three years ago I explained how we can use a couple of undocumented hidden properties of the legend in order to add a legend title (the legend object had no Title property back then – this was only added in a later Matlab release, perhaps as a result of my post). Today I will expand on that article by explaining the plot legend’s internal graphics hierarchy, how we can access each of these components, and then how this information could be used to customize the separate legend components. Note that the discussion today is only relevant for HG2 legends (i.e. R2014b or newer).

Let’s start with a simple Matlab plot with a legend:

hold all; 
hLine1 = plot(1:5); 
hLine2 = plot(2:6); 
hLegend = legend([hLine1,hLine2], 'Location','SouthEast');
hLegend.Title.String = 'MyLegend';

Standard Matlab legend

Standard Matlab legend

This legend is composed of the following visible internal components, which can be customized separately:
Matlab legend components


Id in screenshotAccessed viaObject typeDescriptionImportant properties
1hLegend.TitleTextTitle of the legendVisible, String, Color, FontSize, FontWeight.
2hLegend.TitleSeparatorLineStripSeparator line between title and legend entries. Only appears when title is set.Visible, LineStyle, LineWidth, ColorData (4×1 uint8)
3hLegend.BoxEdgeLineLoopBox (border) line around the entire legend (including title)Visible, LineStyle, LineWidth, ColorData (4×1 uint8)
4hLegend.EntryContainer.NodeChildren(2)LegendEntryEntry row in the legend, corresponding to hLine1Icon, Label, Object (line object in main axes)
5hLegend.EntryContainer.NodeChildren(1)LegendEntryEntry row in the legend, corresponding to hLine2Icon, Label, Object (line object in main axes)
6hLegend.EntryContainer.NodeChildren(1).LabelTextLabel of legend entryVisible, String, Color, FontSize, FontWeight
7hLegend.EntryContainer.NodeChildren(1).IconLegendIconIcon/marker of legend entryVisible, Transform.Children.Children (LineStrip object)

A pivotal object of the legend group are the LegendEntry items, one per legend row:

>> hLegendEntry = hLegend.EntryContainer.NodeChildren(1);
>> get(hLegendEntry)
              Children: [3×1 Graphics]
                 Color: [0 0 0]
                 Dirty: 0
             FontAngle: 'normal'
              FontName: 'Helvetica'
              FontSize: 8
            FontWeight: 'normal'
      HandleVisibility: 'on'
               HitTest: 'on'
                  Icon: [1×1 LegendIcon]
                 Index: 0
           Interpreter: 'tex'
                 Label: [1×1 Text]
            LayoutInfo: [1×1 matlab.graphics.illustration.legend.ItemLayoutInfo]
                Legend: [1×1 Legend]
              Listener: [1×1 event.listener]
                Object: [1×1 Line]
               Overlay: [1×1 TriangleStrip]
          OverlayAlpha: 0.65
                Parent: [1×1 Group]
           PeerVisible: 'on'
         PickableParts: 'visible'
              Selected: 'off'
    SelectionHighlight: 'on'
               Visible: 'on'
       VisibleListener: [1×1 event.proplistener]

Each LegendEntry contains a back-reference to the original graphics object. In my example above, hLegend.EntryContainer.NodeChildren(2).Object == hLine1, and hLegend.EntryContainer.NodeChildren(2).Object == hLine1. Note how the default legend entries order is the reverse of the order of creation of the original graphics objects. Naturally, we can modify this order by creating the legend py passing it an array of handles that is ordered differently (see the documentation of the legend function).

To get all the original graphic objects together, in a single array, we could use one of two mechanisms (note the different order of the returned objects):

% Alternative #1
>> [hLegend.EntryContainer.NodeChildren.Object]'
ans = 
  2×1 Line array:
 
  Line    (data2)
  Line    (data1)
 
% Alternative #2
>> hLegend.PlotChildren
ans = 
  2×1 Line array:
 
  Line    (data1)
  Line    (data2)

For some reason, accessing the displayed graphic line in LegendEntry‘s Icon is not simple. For example, the LineStrip object that corresponds to hLine2 can be gotten via:

hLegendEntry = hLegend.EntryContainer.NodeChildren(1);
hLegendIconLine = hLegendEntry.Icon.Transform.Children.Children;  % a LineStrip object in our example

I assume that this was done to enable non-standard icons for patches and other complex objects (in which case the displayed icon would not necessarily be a LineStrip object). In the case of a line with markers, for example, hLegendIconLine would be an array of 2 objects: a LineStrip object and a separate Marker object. Still, I think that a direct reference in a hLegend.EntryContainer.NodeChildren(1).Icon property would have helped in 99% of all cases, so that we wouldn’t need to pass through the Transform object.

Anyway, once we have this object reference(s), we can modify its/their properties. In the case of a LineStrip this includes LineStyle, LineWidth, ColorData (4×1 uint8), and VertexData (which controls position/length):

>> get(hLegendIconLine(end))  % LineStrip
          AlignVertexCenters: 'on'
             AmbientStrength: 0.3
                ColorBinding: 'object'
                   ColorData: [4×1 uint8]
                   ColorType: 'truecolor'
             DiffuseStrength: 0.6
            HandleVisibility: 'on'
                     HitTest: 'off'
                       Layer: 'middle'
                     LineCap: 'none'
                    LineJoin: 'round'
                   LineStyle: 'solid'
                   LineWidth: 0.5
               NormalBinding: 'none'
                  NormalData: []
                      Parent: [1×1 Group]
               PickableParts: 'visible'
    SpecularColorReflectance: 1
            SpecularExponent: 10
            SpecularStrength: 0.9
                   StripData: []
                     Texture: [0×0 GraphicsPlaceholder]
                  VertexData: [3×2 single]
               VertexIndices: []
                     Visible: 'on'
       WideLineRenderingHint: 'software'

and in the presense of markers:

>> get(hLegendIconLine(1))  % Marker
    EdgeColorBinding: 'object'
       EdgeColorData: [4×1 uint8]
       EdgeColorType: 'truecolor'
    FaceColorBinding: 'object'
       FaceColorData: []
       FaceColorType: 'truecolor'
    HandleVisibility: 'on'
             HitTest: 'off'
               Layer: 'middle'
           LineWidth: 0.5
              Parent: [1×1 Group]
       PickableParts: 'visible'
                Size: 6
         SizeBinding: 'object'
               Style: 'circle'
          VertexData: [3×1 single]
       VertexIndices: []
             Visible: 'on'

An additional undocumented legend property that is of interest is ItemTokenSize. This is a 2-element numeric array specifying the minimal size of the legend entries’ icon and label. By default hLegend.ItemTokenSize == [30,18], but we can either expand or shrink the icons/labels by setting different values. For example:

hLegend.ItemTokenSize == [10,1];  % shrink legend icons and labels

Note that regardless of the amount that we specify, the actual amount that will be used will be such that all legend labels appear.
Fun: try playing with negative values for the icon and the label and see what happens :-)

Have you come across any other interesting undocumented aspect of Matlab legends? If so, then please share it in a comment below.

Related posts:

  1. Plot LineSmoothing property – LineSmoothing is a hidden and undocumented plot line property that creates anti-aliased (smooth unpixelized) lines in Matlab plots...
  2. Multi-column (grid) legend – This article explains how to use undocumented axes listeners for implementing multi-column plot legends...
  3. Controlling plot data-tips – Data-tips are an extremely useful plotting tool that can easily be controlled programmatically....
  4. getundoc – get undocumented object properties – getundoc is a very simple utility that displays the hidden (undocumented) properties of a specified handle object....
  5. Determining axes zoom state – The information of whether or not an axes is zoomed or panned can easily be inferred from an internal undocumented object....
  6. Plot markers transparency and color gradient – Matlab plot-line markers can be customized to have transparency and color gradients. ...
Categories: Handle graphics, Medium risk of breaking in future versions, Undocumented feature

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7 Responses to Plot legend customization

  1. sco1 says:
    July 13, 2018 at 4:40 am

    Very interesting! I never explored NodeChildren when I was building legtools and was using PlotChildren instead to do things like rearrange the legend, add/remove entries, etc.

    Have you had luck utilizing NodeChildren to perform similar options in MATLAB >= R2017a?
    R2017a introduced AutoUpdate to legend objects which broke pretty much all of the functionality I got from manipulating PlotChildren.

    Reply
  2. Peter Cook says:
    July 17, 2018 at 6:17 pm

    I have recently incorporated the Legend’s ItemHitFcn callback property into a utility I built.
    You can access and edit some relevant objects in the callback function, e.g.

    hLegend.ItemHitFcn = myLegendItemHitFcn(src,evnt);
 
% The following in the callback function:
evnt.Peer %access line, patch, image, GraphPlot, etc... 
evnt.Item.Label %access text object for that legend entry
src.String{evnt.Item.Index} = 'new string'; %edit the legend entry name
    Reply
    • Yair Altman says:
      July 18, 2018 at 10:58 am

      @Peter – while the legend object’s ItemHitFcn callback property is documented/supported, the Item property of the eventData object (which is passed to the callback function in run-time when you click a legend entry) is indeed hidden/undocumented. This eventData.Item property contains the LegendEntry item that was clicked. Thanks for pointing it out.

  3. Aaron says:
    March 8, 2019 at 1:38 pm

    It would appear that if you’re using these properties in a function or loop it is important to call drawnow before attempting to utilise any of the LegendEntry objects, otherwise they will not exist yet and you will get errors.

    Reply
  4. Praveen says:
    March 27, 2019 at 7:55 pm

    Hi That’s alright, how do I set for example, marker size in the legend, when I try to set it: hLegend.EntryContainer.NodeChildren(1).Icon.Transform.Children.Children.Size = 15; I get an error saying I can’t set readonly properties, any tips how to go about that. Thanks

    Reply
    • Yair Altman says:
      March 29, 2019 at 4:40 pm

      @Praveen – first note that you have to call drawnow before accessing any of the internal legend components. In addition, note that hLegend.EntryContainer.NodeChildren(1).Icon.Transform.Children.Children returns an array of objects (Marker and LineStrip) and you can only set the Size property of the Marker, not the LineStrip (duh). Here’s a working example:

      figure; hold all;
hLine1 = plot(1:5,'o-b'); 
hLine2 = plot(2:6,'-sr');
hLegend = legend([hLine1,hLine2], 'Location','SouthEast');
hLegend.Title.String = 'MyLegend';
drawnow;
hLegend.EntryContainer.NodeChildren(1).Icon.Transform.Children.Children(1).Size = 15;
  5. Dev says:
    June 25, 2019 at 2:42 pm

    Same question as sco1: Have you had luck utilizing NodeChildren to perform similar options in MATLAB >= R2017a?

    Reply

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