Plotting contourf with discrete x axis

Hello,

I am struggling to plot some data using contourf, and would be extremely grateful for any advice.

My data ("Temp", 10000x10) contains the temperature in 10 vertical "zones" (zone 10 = bottom, hot part, zone 1 = top, cooler part) of a blast furnace, modelled over 10,000 seconds.

I would like to do a contour plot, showing how the temperature changes in all zones vs. time. Ideally, I would get the zone number on the y axis (discrete), the time on the x axis, and the temperature as the colour.

When I plot:

contourf(Temp,50)

I get the following:

MATLAB seems to be doing some sort of interpolation along each "zone" - how can I remove this? The zone (x axis) should be discrete - there is no zone 5.5, just zone 5 and zone 6.

I also can't get the axes to swap despite my best attempts!

Would be extremely grateful for any advice :)

Answer 1

[Kelly Kearney]

The contour function always assumes your input data are points on a surface that can be linearly interpolated between. To get a plot with discrete blocks, you have a few options.

1) Plot using imagesc or pcolor

2) Upsample the data before plotting.

Also, to swap axes, you need to provide x and y inputs or transpose your matrix.

% Data

x1 = 1:10;
y1 = 1:20;

[x3,y3] = meshgrid(x1,y1);

Temp = peaks(20); % Some sample data
Temp = Temp(:,1:10);

% Upsampled data

x2 = linspace(0,10.5,100);
Temp2 = interp1(1:10,Temp',x2,'nearest')';

% Plot options

ax(1) = subplot(2,2,1);
imagesc(x1,y1,Temp);
title('imagesc(Temp)');
hold on;
scatter(x3(:), y3(:), [], Temp(:), 'filled', 'markeredgecolor', 'k');
set(gca, 'ydir', 'normal');

ax(2) = subplot(2,2,2);
pcolor(x1,y1,Temp);
shading flat;
title('pcolor(Temp)');
hold on;
scatter(x3(:), y3(:), [], Temp(:), 'filled', 'markeredgecolor', 'k');
set(gca, 'ydir', 'normal');

ax(3) = subplot(2,2,3);
contourf(x1,y1,Temp,50);
title('contourf(Temp)');
hold on;
scatter(x3(:), y3(:), [], Temp(:), 'filled', 'markeredgecolor', 'k');

ax(4) = subplot(2,2,4);
contourf(x2, y1, Temp2,50);
title('contourf(Temp2)');
hold on;
scatter(x3(:), y3(:), [], Temp(:), 'filled', 'markeredgecolor', 'k');


set(ax, 'clim', [-5 7], 'xlim', [0.5 10.5], 'ylim', [0.5 20.5]);

[G E]

Amazing - thank you very much for the quick reply and detailed explanation!

imagesc exactly what I needed - very happy with it :))