How can I "connect" the colorbar to my values of a scatter plot?

How can I "connect" the colorbar to my values (between 0 and 1) of a scatter plot?

a = rand(1000,1);
C = 1-[a a a];
scatter(1:length(a),a,60,C,'fill')
colormap(C);
colorbar;

Here you can see that the colorbar is not gradually changing from 0 to 1.. How to fix it?

Answer 1

[Star Strider]

I am not certain what you want to do, or what you intend by ‘connect’.

One option —

a = rand(1000,1);
% C = 1-[a a a];
C = ([1;1;1]*linspace(1, 0, 1000)).';
scatter(1:length(a),a,60,C,'fill')
colormap(C);
colorbar;

.

[Sim]

Thanks a lot for youer answer @Star Strider!

With "connect", I mean that, given a certain colormap with a certain number of colors (which change gradually from each other), the values of my scatter plot (i.e. "a = rand(1000,1)", which are between 0 and 1), should get the proper colors from that defined colormap...

About your option, more or less yes, something like that... however, in your colorbar the 1 corresponds to black and 0 corresponds to white, but the scatterplot shows that 1000 corresponds to black... I mean, the variation of color from white to black should go from zero to one, in the y-axis direction, and not in the x-axis direction, right?.....

I was thinking about a solution similar to yours... but I am not sure if it is correct......

a = rand(1000,1);
C = 1-[a a a];
scatter(1:length(a),a,60,C,'fill')
l = 1-linspace(0,1,1000)';
colormap([l l l]) 
colorbar

[Star Strider]

As always, my pleasure!

It would appear to be, if that is the result you want.

You can avoid the interruptions in the axes near the lower end of your plot by by bringing them to the top layer —

a = rand(1000,1);
C = ([1;1;1]*linspace(1, 0, 1000)).';
scatter(a,1:length(a),60,C,'fill')
colormap(C);
colorbar;
Ax = gca;
Ax.Layer = 'top';

That is the only other change I would make to your code if it now does what you want it to do.

.

[Sim]

Hi @Star Strider, I am very sorry, I think I did a mistake to explain my needs, my fault.... :-(

In your figure, the change of color, from white (corresponding to 0) to black (corresponding to 1), goes from value 0 to value 1000. Instead, what I would need is that the change of color, from white (corresponding to 0) to black (corresponding to 1), goes from value 0 to value 1, since my values in

a = rand(1000,1);

range from 0 to 1, and not from 0 to 1000.

In other words, what I would need is something like this (which I think it is not very correct since I use directly the values of "a" as inputs for the color of the scatter plot! Obviouslly, your opinion and the opinion of other users of this Matlab community is very welcome!):

a = rand(1000,1);
C = 1-[a a a];
scatter(1:length(a),a,60,C,'fill')
l = 1-linspace(0,1,1000)';
colormap([l l l]) 
colorbar

Here, we assign a color of the colorbar - which goes from white, at 0, to black, at 1 -, to each value of

a = rand(1000,1);

which values range from 0 to 1.

For example, if we consider the minimum value of "a":

a(285)
ans =
        0.0013207800733116

the corresponding point of the scatter plot should have, basically, a white color, since white corresponds to the value 0 in the colorbar.

As another example,

a(3)
ans =
          0.88899234515285

and the corresponding point of the scatter plot, should have a very dark grey, almost black, since black corresponds to the value 1 in the colorbar.

[Star Strider]

I cannot understand what you want to change.

The colour assignmentts appear to be correct.

I added a few lines to plot specific points with red and green circles, along with ttheir values.

ay = rand(1000,1);
ax = 1:length(ay);
C = 1-[ay ay ay];
scatter(ax,ay,60,C,'fill')
l = linspace(1,0,1000)';
colormap([l l l]) 
colorbar

format shortG
Point_1 = [ax(1) ay(1)]

Point_1 = 1x2

1 0.32606

<mw-icon class=""></mw-icon>

<mw-icon class=""></mw-icon>

Point_1000 = [ax(1000) ay(1000)]

Point_1000 = 1x2

1.0e+00 * 1000 0.94193

<mw-icon class=""></mw-icon>

<mw-icon class=""></mw-icon>

hold on
plot(Point_1(1), Point_1(2), 'or', 'MarkerSize',11)
plot(Point_1000(1), Point_1000(2), 'og', 'MarkerSize',11)
hold off

I will do my best to help you. I need to understand what you want to do.

.

[Sim]

That's great, thanks a lot! Problem solved!

I was not sure about this code:

a = rand(1000,1);
C = 1-[a a a];
scatter(1:length(a),a,60,C,'fill')
l = linspace(1,0,1000)';
colormap([l l l]) 
colorbar

but your further check - with the two red and green points -, confirmed what I was thinking/expecting :-)

Thanks a lot for your great support!

And, I am really sorry for my confusion... My fault :-)

[Star Strider]

As always, my pleasure!

No worries! Often, these sorts of mathematical concepts are difficult to describe.