How do I make the theta-axis or x-axis change color?

Hi everyone,

I am trying to plot a function that takes in values of hues. It would be great if I can dye my theta-axis in polar plot or x-axis in plot according to the hue values. Namely I want my axis to have 256 colors all at once. This was I can easily visualize what color corresponds to what r or y value. Is there a way to do so please?

Thanks, J.

Answer 1

[Star Strider]

This is as close as I can get to doing what you want:

x = linspace(0, 2*pi, 256);
y = sin(x).^2;

figure(1)
plot(x, y)
hold on
wdth = 0.005;
hp = patch([x fliplr(x)], [ones(size(x)) -ones(size(x))]*wdth, [0:255 255:-1:0]*100);
set(hp, 'EdgeColor','none')
hold off
colormap(jet(256))
grid

Experiment to get the result you want. The ‘wdth’ variable is the width of the x-axis line.

The patch object will not work with polar axes using polarplot. Apparently, that is one of the polarplot restrictions.

[Jack Z]

Yeah I think it does the job! Could you help me understand what the patch function is doing please? Thanks!

I know that it defines a polygon, but not sure how it defines the shape, location, or color.

[Star Strider]

As always, my pleasure!

The patch (link) function here defines a very narrow rectangle with x-coordinates defined by ‘[x fliplr(x)]’, so it goes up and then back down, and with y-coordinates [ones(size(x)) -ones(size(x))]*wdth tracing the rectangle. That defines the shape of the line with ‘wdth’ being its width. The colours are defined here by a vector going from 0 to 25500 and back to 0, because every segment of the patch has to have a specific colour here. (The 25500 value is the result of an experiment that worked. It has to be the same size as the x- and y-coordinates.) As far as the location, I defined it to overplot the x-axis by defining the rectangle to be above and below it. It did not obscure the axis ticks or labels or gridlines when I ran it. You can vary its width by changing the ‘wdth’ variable, and shift it up or down by adding a constant to it.

I could not use the patch object in polarplot. I tried with pol2cart but was not successful. One approach I just now thought of is to use code similar to the code I used to define a colour wheel a while ago in Colormap on the outside of a circle (link). You might be able to adapt that code to use with your pol2cart plot of your function.

[Jack Z]

Awesome! Thanks a lot!

[Star Strider]

As always, my pleasure!

Answer 2

[Adam]

hAxes.XAxis.Color = 'b';

will, for example turn the x axis of an axes handle, hAxes, blue. Obviously you can use an RGB triplet also for more accurate colours. This is only valid for relatively recent Matlab versions, R2015b or later, though.

[Jack Z]

Thanks Adam. The thing is I actually want more than one color on my axis. I need 256 colors to represent my x or theta values. Is there a way to do so?

[Adam]

I'm not aware that you can colour axes multiple colours, but you can create a 'line' along the axes that is multicolour, though also as far as I am aware this cannot be done with a standard line and would have to be done as a 'surface' object (or many joined lines, but that would not be very performant)

[Jack Z]

Thanks Adam!