Hello, I want to plot two variables (with different number of phases)...

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Jessica Webb on 20 Sep 2017
Answered: Jessica Webb on 21 Sep 2017
Hello, I have two columns of data (one volume with 13 points, and the other pressure with 60 points).
I ultimately want to plot them against each other (they both start at 0 and finish at 400ms) but also want to know the exact volume and pressure for any time point between 0 and 400ms.
I am a matlab novice and so any help would be wonderful Thank you

Simon Henin on 21 Sep 2017
To plot your data against one another you could use plotyy
% make up some data
volume = [1:13]'+wgn(13,1, 10); % add some wgn noise to the simulated data
pressure = [1:60]'+wgn(60,1, 10);
% create time vectors, assuming each variable is linearly spaced (this may
% not be the case, but just an assumption)
t1 = linspace(0,400, 13)';
t2 = linspace(0,400, 60)';
% plot them against each other
ax = plotyy(t1, volume,t2, pressure); hold on;
ylabel(ax(1), 'Volume');
ylabel(ax(2), 'Pressure');
legend({'Volume', 'Pressure'});
Without knowing more information about the time vs. amplitude relationship of your data, it is hard to give you a direct answer as to how to interpolate your results to any given time point. However, if your data have a linear relationship (e.g. volume increases linearly as a function of time), then you could do a simple regression to get a best fit to your data, and interpolate from the result:
% If your variables have a linear relationship, them find a regressor that
% create a vector of all times of interest (e.g. 0-400ms, in 1ms steps)
t3 = linspace(0,400, 401);
% find regression line that fits the data
X = [ones(length(volume),1) t1];
b = X\volume;
y = b*t3;
figure;
plot(t1, volume, 'o'); hold on;
plot(t3,y(2,:)); % plot the slope+intercept
plot(t3(100), y(2,100), '^', 'markersize', 18);
legend({'Volume', 'Regression Fit to Data', 'Data fit at t=100ms'});
Walter Roberson on 21 Sep 2017
Instead of regression, you can use interp1:
starttime = 1;
endtime = 400;
timebase_v = linspace(starttime, endtime, length(volume));
timebase_p = linspace(starttime, endtime, length(pressure));
common_timebase = starttime : endtime;
interp_v = interp1(timebase_v, volume, common_timebase);
interp_p = interp1(timebase_p, pressure, common_timebase);
plot(common_timebase, interp_v, 'r', common_timebase, interp_p, 'b');
legend({'volume', 'pressure'})

Jessica Webb on 21 Sep 2017
Thanks for this, will take a look. Really appreciate it!