Colorbar not plotting the right colours

Asked by Raymond Hoang on 30 Oct 2016

Latest activity Commented on by Image Analyst on 31 Oct 2016

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I'm having trouble using the colorbar/colormap functions. I create a test plot, plotting a bar graph from 0,0 to 100,100 and created a color bar. I would like the color bar to correspond to the y values of the graph. Which means, if looking at the xaxis, the graph would be colored in the same order as the colorbar from 0->100 by matching the y-values to the colorbar values.

It seems to be plotting the value 0 corresponding to the colorbar which is purple/black every time which I believe is the z-value. Mainly see colorbar/maps used for contour plots but I would like it to work for a 2D plot.

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Answer by Image Analyst on 31 Oct 2016

Edited by Image Analyst on 31 Oct 2016

Consider creating an indexed image instead of a bar chart. Then you can write each bar as a column with the gray level of the color you want. Apply the colormap and that column of pixels will show up in the desired color.

If you want a color gradient in each bar, then that is more complicated - you'd have to write a ramp of pixel levels rather than a constant pixel level that corresponds to the tallest color.

rows = 100;
columns = 100;
indexedImage = zeros(rows, columns, 'uint8');
for col = 1 : 100
  row1 = rows-col+1
  indexedImage(row1:rows, col) = col;
end
imshow(indexedImage);
colormap(gca, jet(rows));
caxis([0, rows]);
colorbar;

Modify as needed.

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Answer by Marc Jakobi on 31 Oct 2016

You cannot create a colorbar for bar() graphs in Matlab. What you can do is use a loop to create a separate bar graph for each color.

See this answer for an example.

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Raymond Hoang on 31 Oct 2016

I'm not sure how to incorporate that answer with what I'm trying to do.

I have a function that is constantly outputting a value "y" but not putting it into a matrix form.

figure(1)
hold on
ylabel 'Temperature (Degrees)'
title 'Live Temperature Plot'
i = 0;
while true
      [t] = ReadData(out)
      figure(1)
          if(i <= 100)
              xlim([0,200]);ylim([0,100])
          else
              xlim([i-200, i]);ylim([0,100])
          end
      tempplot = bar(i,tc,'BarWidth',1);
      i = i + 1;
      pause(0.05)
  end

See this link at 6:50

Image Analyst on 31 Oct 2016

barchart.m

I'll show you. See my attached demo.

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