Colouring Bars by 'Y' value

Hello Community,

I am trying to plot a bar chart and to colour the bars according to a rule related to the values for the 'Y' axis. So, if the Y value is a minus figure, the bar should be coloured grey, and if positive, should be coloured black. I have tried the following (and several variations):

% Colouring bars
if y <0;
    bar(y,'RGB::Grey');
elseif y >= 0;
    bar(y,'RGB::Black');
end

but with no luck. Everything else about the plotting that I am doing works fine.

I'm sure this will be simple for the more experienced, so could anyone suggest a fix to help me please?

Thank you,

10B.

Answer 1

[Mike Garrity]

I would suggest the approach I described in this blog post. In this case, it would look something like this:

x = 1:18;
y = randn(1,18);
mask = y > 0;
y1 = y;
y1(mask) = nan;
bar(x,y1,'FaceColor','red')
hold on
y2 = y;
y2(~mask) = nan;
bar(x,y2,'FaceColor','green')

[Image Analyst]

Mike, I think the best way would be if bar() took an argument that specified the color of each bar. Currently the color you send in applies to all bars. What it should do is to take an array, just like scatter() does, which is an array of colors so that you can specify the color of each, just like scatter() already does. So if you send in only one color, then all bars are that color but if you send in an N-by-3 array, it will give those colors to the corresponding N bars. Do you think they could make that improvement, to bring it into alignment with what scatter already does?

[Mike Garrity]

Yeah, that's on our list, but I'm afraid it's not very close to the top of the list.

[10B]

Hello Mike,

Thanks for your informative answer and link to the blog information (hadn't seen that article).

To be fair, I have accepted your answer, but Star Strider's answer helped as well as this gave me the colouring options that I wanted, so I merged the best parts of the two to get the following:

figure('Color',[1 1 1]);
cm = [0.90 0.90 0.90; 0 0 0]; 
mask = y > 0;
y1 = y;
y1(mask) = nan;
bar(x,y1,'FaceColor',cm(1,:))
hold on
y2 = y;
y2(~mask) = nan;
bar(x,y2,'FaceColor',cm(2,:))
hold off

With only a couple of tweaks to the colouring values 'cm'.

This now does exactly what I want - Thanks!

Answer 2

[Star Strider]

This seems to work, at least on my test data:

x = 1:10;
y = randi([-9 9], 1, 10);

yp = y>0;                                       % Logial Indices: Positive ‘y’
yn = y<0;                                       % Logial Indices: Negative ‘y’
cm = [0.5 0.5 0.5; 0 0 0];                      % Colour Matrix

figure(1)
bar(x(yn), y(yn), 'FaceColor',cm(1,:))
hold on
bar(x(yp), y(yp), 'FaceColor',cm(2,:))
hold off
set(gca, 'XTick', x)

[10B]

Excellent information Star Strider - thanks a lot for your help. You will see from the comment above that I merged the two bits of code to get what I wanted, and yours in particular helped with the correct colouring.

Thanks very much!

[Star Strider]

My pleasure!

A Vote would be appreciated!