Plot markers transparency and color gradient

Last week I explained how to customize plot-lines with transparency and color gradient. Today I wish to show how we can achieve similar effects with plot markers. Note that this discussion (like the preceding several posts) deal exclusively with HG2, Matlab’s new graphics system starting with R2014b (well yes, we can also turn HG2 on in earlier releases).

As Paul has noted in a comment last week, we cannot simply set a 4th (alpha transparency) element to the MarkerFaceColor and MarkerEdgeColor properties:

>> x=1:10; y=10*x; hLine=plot(x,y,'o-');
>> hLine.MarkerFaceColor = [0.5,0.5,0.5];      % This is ok
>> hLine.MarkerFaceColor = [0.5,0.5,0.5,0.3];  % Not ok
While setting the 'MarkerFaceColor' property of Line:
Color value must be a 3 element numeric vector

Standard Matlab plot markers

Lost cause? – not in a long shot. We simply need to be a bit more persuasive, using the hidden MarkerHandle property:

>> hMarkers = hLine.MarkerHandle;  % a matlab.graphics.primitive.world.Marker object
 
>> hMarkers.get
    EdgeColorBinding: 'object'
       EdgeColorData: [4x1 uint8]
       EdgeColorType: 'truecolor'
    FaceColorBinding: 'object'
       FaceColorData: [4x1 uint8]
       FaceColorType: 'truecolor'
    HandleVisibility: 'off'
             HitTest: 'off'
               Layer: 'middle'
           LineWidth: 0.5
              Parent: [1x1 Line]
       PickableParts: 'visible'
                Size: 6
               Style: 'circle'
          VertexData: [3x10 single]
       VertexIndices: []
             Visible: 'on'
 
>> hMarkers.EdgeColorData'  % 4-element uint8 array
ans =
    0  114  189  255
 
>> hMarkers.FaceColorData'  % 4-element uint8 array
ans =
  128  128  128  255

As we can see, we can separately attach transparency values to the marker’s edges and/or faces. For example:

hMarkers.FaceColorData = uint8(255*[1;0;0;0.3]);  % Alpha=0.3 => 70% transparent red

70% Transparent Matlab plot markers

And as we have seen last week, we can also apply color gradient across the markers, by modifying the EdgeColorBinding/FaceColorBinding from ‘object’ to ‘interpolated’ (there are also ‘discrete’ and ‘none’), along with changing the corresponding FaceColorData/EdgeColorData from being a 4×1 array to a 4xN array:

>> colorData = uint8([210:5:255; 0:28:252; [0:10:50,40:-10:10]; 200:-10:110])
colorData =
  210  215  220  225  230  235  240  245  250  255
    0   28   56   84  112  140  168  196  224  252
    0   10   20   30   40   50   40   30   20   10
  200  190  180  170  160  150  140  130  120  110
 
>> set(hMarkers,'FaceColorBinding','interpolated', 'FaceColorData',colorData)

Matlab plot markers with color and transparency gradients

This can be useful for plotting comet trails, radar/sonar tracks, travel trajectories, etc. We can also use it to overlay meta-data information, such as buy/sell indications on a financial time-series plot. In fact, it opens up Matlab plots to a whole new spectrum of customizations that were more difficult (although not impossible) to achieve earlier.

Throughout today, we’ve kept the default FaceColorType/EdgeColorType value of ‘truecolor’ (which is really the same as ‘truecoloralpha’ as far as I can tell, since both accept an alpha transparency value as the 4th color element). If you’re into experimentation, you might also try ‘colormapped’ and ‘texturemapped’.

1. Plot line transparency and color gradient Static and interpolated (gradient) colors and transparency can be set for plot lines in HG2. ...
2. Plot LimInclude properties The plot objects' XLimInclude, YLimInclude, ZLimInclude, ALimInclude and CLimInclude properties are an important feature, that has both functional and performance implications....
3. Draggable plot data-tips Matlab's standard plot data-tips can be customized to enable dragging, without being limitted to be adjacent to their data-point. ...
4. Undocumented scatter plot jitter Matlab's scatter plot can automatically jitter data to enable better visualization of distribution density. ...