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Customizing histogram plots

Posted By Yair Altman On January 17, 2018 | __No Comments__

Earlier today, I was given the task of displaying a histogram plot of a list of values. In today’s post, I will walk through a few customizations that can be done to bar plots and histograms in order to achieve the desired results.

We start by binning the raw data into pre-selected bins. This can easily be done using the builtin * histc* (deprecated) or

```
[binCounts, binEdges] = histcounts(data);
hBars = bar(hAxes, binEdges(1:end-1), binCounts);
```

Let’s improve the appearance: In my specific case, the data was financial return (percentage) values, so let’s modify the x-label format accordingly and display a title. To make the labels and title more legible, we decrease the axes

```
hAxes = hBar.Parent;
xtickformat(hAxes, '%g%%');
title(hAxes, 'Distribution of total returns (monthly %)');
set(hAxes, 'FontSize',8, 'Box','off')
```

So far nothing undocumented. Note that the

Now, let’s use a couple of undocumented properties: to remove the excess white-space margin around the axes we’ll set the axes’

```
set(hAxes, 'LooseInset',[0,0,0,0]); % default = [.13,.11,.095,.075]
hAxes.XRuler.TickLabelGapOffset = -2; % default = +2
```

Note that I used the undocumented axes

Now let’s take a look at the bin labels: The problem with the bar plot above is that it’s not intuitively clear whether the bin for “5%”, for example, includes data between 4.5-5.5 or between 5.0-6.0 (which is the correct answer). It would be nicer if the labels were matched to the actual bin edges. There are 3 basic ways to fix this:

- We could modify the bar plot axes tick values and labels, in essence “cheating” by moving the tick labels half a bin leftward of their tick values (don’t forget to add the extra tick label on the right):
hAxes.XTick(end+1) = hAxes.XTick(end) + 1; % extra tick label on the right labels = hAxes.XTickLabels; % preserve tick labels for later use below hAxes.XTick = hAxes.XTick - 0.5; % move tick labels 1/2 bin leftward hAxes.XTickLabel = labels; % restore pre-saved tick labels hAxes.XLim = hAxes.XLim - 0.5; % ...and align the XLim

- We could use the
function’s optional**bar**`'histc'`

flag, in order to display the bars in histogram mode. The problem in histogram mode is that while the labels are now placed correctly, the bars touch each other – I personally find distinct bars that are separated by a small gap easier to understand.hBars = bar(..., 'histc'); % [snip] - same customizations to hAxes as done above

With the original bar chart we could use the built-in**BarWidth**to set the bar/gap width (default: 0.8 meaning a 10% gap on either side of the bar). Unfortunately, callingwith**bar**`'hist'`

or`'histc'`

(i.e. histogram mode) results in a`Patch`

(not`Bar`

) object, and patches do not have a**BarWidth**property. However, we can modify the resulting patch vertices in order to achieve the same effect:% Modify the patch vertices (5 vertices per bar, row-based) hBars.Vertices(:,1) = hBars.Vertices(:,1) + 0.1; hBars.Vertices(4:5:end,1) = hBars.Vertices(4:5:end,1) - 0.2; hBars.Vertices(5:5:end,1) = hBars.Vertices(5:5:end,1) - 0.2; % Align the bars & labels at the center of the axes hAxes.XLim = hAxes.XLim + 0.5;

This now looks the same as option #1 above, except that the top-level handle is a

`Patch`

(not`Bar`

) object. For various additional customizations, either`Patch`

or`Bar`

might be preferable, so you have a choice.

- Lastly, we could have used the builtin
function**histogram**^{[5]}instead of. This function also displays a plot with touching bars, as above, using**bar**`Quadrilateral`

objects (a close relative of`Patch`

). The solution here is very similar to option #2 above, but we need to dig a bit harder to modify the patch faces, since their vertices is not exposed as a public property of the`Histogram`

object. To modify the vertices, we first get the private**Face**property (explanation^{[6]}), and then modify its vertices, keeping in mind that in this specific case the bars have 4 vertices per bar and a different vertices matrix orientation:hBars = histogram(data, 'FaceAlpha',1.0, 'EdgeColor','none'); % [snip] - same customizations to hAxes as done above % Get access to *ALL* the object's properties oldWarn = warning('off','MATLAB:structOnObject'); warning off MATLAB:hg:EraseModeIgnored hBarsStruct = struct(hBars); warning(oldWarn); % Modify the patch vertices (4 vertices per bar, column-based) drawnow; % this is important, won't work without this! hFace = hBarsStruct.Face; % a Quadrilateral object (matlab.graphics.primitive.world.Quadrilateral) hFace.VertexData(1,:) = hFace.VertexData(1,:) + 0.1; hFace.VertexData(1,3:4:end) = hFace.VertexData(1,3:4:end) - 0.2; hFace.VertexData(1,4:4:end) = hFace.VertexData(1,4:4:end) - 0.2;

In conclusion, there are many different ways to improve the appearance of charts in Matlab. Even if at first glance it may seem that some visualization function does not have the requested customization property or feature, a little digging will often find either a relevant undocumented property, or an internal object whose properties could be modified. If you need assistance with customizing your charts for improved functionality and appearance, then consider contacting me for a consulting session ^{[7]}.

Categories: Handle graphics, Medium risk of breaking in future versions, Stock Matlab function, Undocumented feature

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URL to article: **https://undocumentedmatlab.com/articles/customizing-histogram-plots**

URLs in this post:

[1] * bar* function:

[2] see this post: **http://undocumentedmatlab.com/blog/setting-axes-tick-labels-format**

[3] **LooseInset** property: **http://undocumentedmatlab.com/blog/axes-looseinset-property**

[4] **XAxis** property: **https://www.mathworks.com/help/matlab/ref/matlab.graphics.axis.axes-properties.html#prop_XAxis**

[5] * histogram* function:

[6] explanation: **http://undocumentedmatlab.com/blog/accessing-private-object-properties**

[7] consulting session: **http://undocumentedmatlab.com/consulting**

[8] Customizing contour plots : **https://undocumentedmatlab.com/articles/customizing-contour-plots**

[9] Customizing contour plots part 2 : **https://undocumentedmatlab.com/articles/customizing-contour-plots-part2**

[10] Customizing contour plots part 2 : **https://undocumentedmatlab.com/articles/customizing-contour-plots-part-2**

[11] Customizing axes tick labels : **https://undocumentedmatlab.com/articles/customizing-axes-tick-labels**

[12] Customizing axes part 3 – Backdrop : **https://undocumentedmatlab.com/articles/customizing-axes-part-3-backdrop**

[13] Customizing axes part 4 – additional properties : **https://undocumentedmatlab.com/articles/customizing-axes-part-4-additional-properties**

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