MATLAB: Define vector to use for colorbar

colorbar; colormap; scatter plot; bar plotMATLAB

Hi,

I am plotting a bar+scatter plot where the scatter points are colored according to a separate variable. The problem I am having is that the colorbar is, for some reason that is beyond me, using the wrong values at the moment. If I just plot the scatter plot and add a colorbar, then the range of the colorbar is correct. I have been trying every solution I can come up with, but have not made any progress. Any help would be very much appreciated. Please find a working example of the code below;

figure
          subplot(2,1,1)
          a = 1;
          b = 2;
          r = (b-a).*rand(1,7) + a;
          y = r;
          rr = (b-a).*rand(1,7) + a;
          z = rr;
          x = [1:7];
          zz = rand(1,7)
          yyaxis left
          hold on
          for i = 1:7
              h=bar(i,y(i), 'FaceColor',[1 1 1], 'LineWidth',3);
              yb(i) = cat(1, h.YData);
              xb(i) = bsxfun(@plus, h(1).XData, [h.XOffset]');
              if zz(i) < 0.0300000
                  set(h,'EdgeColor','k');
              elseif zz(i) < 0.050000000
                  set(h,'EdgeColor','b');
              elseif zz(i) < 0.070000000
                  set(h,'EdgeColor','g');
              else
                  set(h,'EdgeColor','r');
              end
          end
          ylabel('hm', 'FontSize', 12, 'FontWeight', 'bold')
      for i1=1:7
              t = text(xb(i1)-0.2,yb(i1),num2str(yb(i1),'%0.3f'),...
                         'HorizontalAlignment','center',...
                         'VerticalAlignment','bottom')
                          s = t.FontSize;
                          t.FontSize = 12;
                          t.FontWeight = 'bold';
      end
          yyaxis right
          pointsize = 40;
          hh = scatter(x,z,pointsize, zz,'filled')
          cc = colormap([hsv(20)])
          c = colorbar
          c.Label.String = 'Pos';
          set(gca,'Ydir','reverse')
          ylabel('OK', 'FontSize', 12, 'FontWeight', 'bold')
          lgd = legend([h, hh], 'hm', 'OK')
      subplot(2,1,2)
      x = [1:8]
      a = 1;
      b = 2;
      r = (b-a).*rand(1,8) + a;
      y = r;
      rr = (b-a).*rand(1,8) + a;
      z = rr;
      zz = rand(1,8);
      yyaxis left
      hold on
      for i = 1:8
              h=bar(i,y(i), 'FaceColor',[1 1 1], 'LineWidth',3);
              yb(i) = cat(1, h.YData);
              xb(i) = bsxfun(@plus, h(1).XData, [h.XOffset]');
              if zz(i) < 0.0300000
                  set(h,'EdgeColor','k');
              elseif zz(i) < 0.050000000
                  set(h,'EdgeColor','b');
              elseif zz(i) < 0.070000000
                  set(h,'EdgeColor','g');
              else
                  set(h,'EdgeColor','r');
              end
      end
      for i1=1:8
              t = text(xb(i1)-0.2,yb(i1),num2str(yb(i1),'%0.3f'),...
                         'HorizontalAlignment','center',...
                         'VerticalAlignment','bottom')
                          s = t.FontSize;
                          t.FontSize = 12;
                          t.FontWeight = 'bold';
      end
      ylabel('hm', 'FontSize', 12, 'FontWeight', 'bold')
      yyaxis right
      pointsize = 40;
      hh = scatter(x,z,pointsize, zz,'filled')
      set(gca,'Ydir','reverse')
      ylabel('OK', 'FontSize', 12, 'FontWeight', 'bold')
      c = colorbar
      c.Label.String = 'Pos';
      lgd = legend([h, hh], 'hm', 'OK')
          %title(lgd,'My Legend Title')
          hold off

Thanks in anticipation.

Regards Joel

Best Answer

Ok, I spend some time with this and I think i see what's happening. It all boils down to the way you define the CData property of your scatter plot. Here is a simple example that should illustrate how it works:

    xData = 1:10
    yData = rand(1, 10) + 1 % random number between 1 and 2
    cVect = yData;
    colormap(jet(10));
    hScat = scatter(xData, yData, 100, cVect, 'Filled')
    hcbar = colorbar
    caxis([1 2])

This sets the CData to the same value as the yData vector, and then we use caxis to determine the boundaries of the displayed color scale.

Now in your code, it looks like you are using the variable zz as your CData value in your scatter plot. You define it this way:

zz = rand(1,7)

This means it will be a 7 element vector with values between 0 and 1. When you use colormap you are creating an array of colors that your CData will be mapped to. Without specifying a caxis, I think Matlab automatically maps the bottom color to the lowest CData value, and the top color to the highest CData value.

From your code, I assume you want to map a zz value of 0 to the bottom color, and a zz value of 1 to the top color?

If so, the following simplification of your example should do the trick:

figure
  % Generate Data
      a = 1;
      b = 2;
      r = (b-a).*rand(1,7) + a;
      y = r;
      rr = (b-a).*rand(1,7) + a;
      z = rr;
      x = [1:7];
      zz = rand(1,7)
    % yyaxis left % Removed because I'm on 2014b
    % Draw rectangles    
    hold on
    for i = 1:7
        hBarGraph = bar(i,y(i), 'FaceColor',[1 1 1], 'LineWidth',3);
        yb(i) = cat(1, hBarGraph.YData);
        xb(i) = bsxfun(@plus, hBarGraph(1).XData, [hBarGraph.XOffset]');
        if zz(i) < 0.0300000
            set(hBarGraph,'EdgeColor','k');
        elseif zz(i) < 0.050000000
            set(hBarGraph,'EdgeColor','b');
        elseif zz(i) < 0.070000000
            set(hBarGraph,'EdgeColor','g');
        else
            set(hBarGraph,'EdgeColor','r');
        end
    end
    ylabel('hm', 'FontSize', 12, 'FontWeight', 'bold')
    % Text labels for rectangles
    for i1=1:7
        t = text(xb(i1)-0.2,yb(i1),num2str(yb(i1),'%0.3f'),...
                'HorizontalAlignment',  'center',...
                'VerticalAlignment',    'bottom');
                t.FontSize = 12;
                t.FontWeight = 'bold';
    end
    % yyaxis right % Removed because I'm on 2014b
  % Scatter Plot
    pointsize = 40;
    hAxes = gca;
    colormap(hsv(20))
    colorBounds = [0 1];
    caxis(colorBounds)
    colorbar
    hScatterPlot = scatter(x,z,pointsize, zz,'filled')
    set(hAxes,'Ydir','reverse')
    ylabel('OK', 'FontSize', 12, 'FontWeight', 'bold')
    lgd = legend([hBarGraph, hScatterPlot], 'hm', 'OK')

If this doesn't help, please let us know more about what color behavior you want. Good luck, and let us know how it goes! :)

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Background info

scatter & scatter3 use the axes' colormap along with the color axis limit (caxis|ax.CLim) to set the color of the markers.

To get the color of marker at index n, scale the scatter object's CData which contains the color scale value to the range of the colormap.

Solution

Use an anonymous function that receives an index value and returns the RGB vector associated with that data point. Set the colormap, color limit (caxis|ax.CLim) and the scatter object before creating the anonymous function.

ax = gca(); 
h = scatter3(___);
cmap  = ax.Colormap; 
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% Example: Get RGB values for 4th data point in h

% getRGB(4)
%  ans = [0.2576 0.1814 0.7501]

Alternatively the axis and scatter handles could be included as inputs if you expect them to change.

getRGB = @(i,h,ax)ax.Colormap(round((h.CData(i)-ax.CLim(1))/range(ax.CLim) * (size(ax.Colormap,1)-1) + 1),:); 
% Example: Get RGB values for 4th data point in h
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%  ans = [0.2576 0.1814 0.7501]

Demo

Circle markers are added to verify that their color matches the original data.

% Set up scatter3 plot
x = 0 : 0.1 : 10;
y = sin(x) + 1;
z = sin(y).^2;      %added '.'
point_size = 2*ones(size(x));
my_color = x;
h = scatter3(x,y,z, point_size, my_color); % added handle output
colorbar            % added
% Create a function to return the RGB value given an index value
ax = gca(); 
cmap  = ax.Colormap; 
clim = caxis(ax);
% create function: rgb = getRGB(i) 
% Create this after creating the scatter3 object, colormap, and color limits.  
getRGB = @(i)cmap(round((h.CData(i)-clim(1))/range(clim) * (size(cmap,1)-1) + 1),:);
% Test it by adding o markers around each point, matching colors
hold on
for i = 1:numel(h.XData)
    plot3(h.XData(i), h.YData(i), h.ZData(i), 'o','Color', getRGB(i))
end
●

MATLAB: How can i use legend command when i am using hold on for plotting graphs? in the plot i want to use legend command to indicate the color for respective plot that is black for low pass,blue for high pass,green for power complementary,red for all pas

Use the legend function after the three plots

close all
k = input('Number of frequency points = ');     %%256
w = 0:pi/k:pi;
alpha=0.3
num=((1-alpha)/2)*[1 1]
den=[1 -alpha]
hlp=freqz(num,den,w)
num1=((1+alpha)/2)*[1 -1]
den1=[1 -alpha]
hhp=freqz(num1,den1,w)
figure
subplot(2,1,1)
plot(w/pi, abs(hlp),'k','LineWidth', 2);hold on
grid on;
xlabel('\omega/\pi', 'FontSize', 12, 'FontWeight', 'Bold')
ylabel('Amplitude', 'FontSize', 12, 'FontWeight', 'Bold')
title('|H(\omega)|', 'FontSize', 12, 'FontWeight', 'Bold')
set(gca, 'FontSize', 14, 'FontWeight', 'Bold')
plot(w/pi, abs(hhp), 'LineWidth', 2);hold on
grid on;
xlabel('\omega/\pi', 'FontSize', 12, 'FontWeight', 'Bold')
ylabel('Amplitude', 'FontSize', 12, 'FontWeight', 'Bold')
title('|H(\omega)|', 'FontSize', 12, 'FontWeight', 'Bold')
set(gca, 'FontSize', 14, 'FontWeight', 'Bold')
plot(w/pi, abs(all_pass),'r', 'LineWidth', 2);hold on
grid on;
xlabel('\omega/\pi', 'FontSize', 12, 'FontWeight', 'Bold')
ylabel('Amplitude', 'FontSize', 12, 'FontWeight', 'Bold')
title('|H(\omega)|', 'FontSize', 12, 'FontWeight', 'Bold')
set(gca, 'FontSize', 14, 'FontWeight', 'Bold')
legend ('low pass','high pass','all pass','Location', 'Northwest')
set(legend,'FontSize',7);

Best Answer

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MATLAB: Convert point from color map / color bar to color vector

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