Highlight specific entry in matrix heatmap

Given the matrix

A = rand(10,10);

We can visualize this with

heatmap(A);

I would like to "highlight" some fixed entries of A regardless of their values. For example, A(1,1) and A(3,2) as in the image

<<

>>

. One way would be to place an "X" on the specific values. Another would be to make the border around these entries bold.

How could one go about this efficiently?

Answer 1

[Walter Roberson]

You appear to be using the new heatmap() call, not the older biograph HeatMap() call.

Unfortunately, the new heatmap objects cannot have their text or decoration customized on a per-square basis.

You would need to figure out where the squares were on the display and text() the marking into place.

[matpylab]

Do you know if it can be easily achieved with other functions such as

imagesc(A);
colorbar;

[Walter Roberson]

Yes, it can be done. The most difficult part is figuring out the logic that was used to create the color map. It appears to me that it might have been constructed in HSV, with hue = 0.56612685560054 for each, with saturation mostly linear from 0.0760702186633815 to 1, and value quite linear from 0.9741 to 0.741

Once the colormap has been created, you can imagesc() and colormap() and colorbar(). set Xtick and YTick. Then you can text() any labels into place.

Note: the center of the upper left box is at (1,1), and the center of the lower right box is at (10,10). You can therefore use the nice integers to specify the location of your text -- but make sure that you set vertical and horizontal alignment to be 'center' so that the text you move in is centered in the square.

[Benjamin Kraus]

You can query the Colormap property on the heatmap to get the default colormap, but it was created using a fairly simple algorithm:

m = 256; % Desired number of colors
blue = [0 0.4470 0.7410]; % First color in ColorOrder
a = linspace(0.1,1,m)';
map = blue.*a + (1 - a);