Change colour of points in plot3 with increasing z value

Hi I have a set of x values, set of y values and for each x,y I have a z value. I have to plot them as shown in the attached figure. I am using plot3 to plot that graph. I want to change the colour of the points according to the value of z. The more the value of z, more is the intensity. Any help will be appreciated.

Answer 1

[Mike Garrity]

No, plot3 only supports a single color. You can use other graphics functions. The "traditional" one for this purpose is actually rather surprising. It's the patch function, which is designed for drawing filled polygons.

Consider this example:

x = cos(theta);
y = sin(theta);
z = theta;
plot3(x,y,z)

I can draw it colored by Z like this:

cla
patch([x nan],[y nan],[z nan],[z nan],'EdgeColor','interp','FaceColor','none')

What's going on here is the following:

• The 4th arg says to use Z for color data
• The EdgeColor=interp says to interpolate the color data as the edge color
• The FaceColor=none says to not fill the polygon. Just draw the edges
• The nans say not to connect the last point to the first point.

There are a couple of other options, but that's probably what most old-school MATLAB programmers use.

[Mahi Nazir]

Thanks Mike. I want discrete data points in z, so I use plot3(x, y, z, '*') How will I get these discrete points using patch?

Answer 2

[Steven Lord]

Based on the picture, which did not have lines connecting the points, I think SCATTER3 is the function you're looking for.

[Mike Garrity]

Yes, Steve's right. That's the better bet.

That'll teach me to not look at the picture!

[Mahi Nazir]

Thank you Steven. How to set the colour intensity based on the value of z. If say, z= 8 for both (x1, y1) and say (x3, y3), z should be plotted in the same colour when z has the same value. So that the plot is colour coded. Can you cite an example with the kind of data I have please. My x is (nx1), y is (nx1) and z obviously is (nx1).

[John Dzielski]

Hi,

This example does not work in 2023b. The syntax [x nan] now produces an error. I tried using [x, nan(size(x))] and that produces an almost correct plot. The feature that is missing is that the first and last points are connected. Thanks.

[Voss]

@John Dzielski: It could be that your x, y, and z are column vectors (instead of row vectors as in the example), in which case you'd use the syntax [x; nan] to vectically concatenate x with a NaN (instead of [x nan] to horizontally concatenate x with a NaN)

theta = linspace(0,4*pi,100).';
x = cos(theta);
y = sin(theta);
z = theta;
whos x y z % column vectors

  Name        Size            Bytes  Class     Attributes

  x         100x1               800  double              
  y         100x1               800  double              
  z         100x1               800  double              

patch([x; nan],[y; nan],[z; nan],[z; nan],'EdgeColor','interp','FaceColor','none')
view(3)