Matlab - finding x-value for known y-value from a linear plot?

Factual Questions
1

Couldn’t really think of the best title for this thread, so if anyone can come up with something that describes the below better, please suggest!

Anyway, not used Matlab for quite a while, can just about remember the basics but I’m doing calculations based on data from a wind-tunnel lab I did recently.

The background isn’t that important, but I’ve got a load cell, and I want to manipulate the calculation data. I’ve managed to plot three sets of loading different loads against voltage data, taken the average then plotted a least-squares fit with polyval.

That’s fine, but I want to use this to find the force for a given voltage now.

So what matlab function do I use to do the reverse of plotting, and find the x value for a given y value on this new line (function) I’ve created?

I can’t really think of the name for doing this, which means I’m finding it hard to search for help on the internet!

Thanks in advance.

2

What you’re trying to do is linear interpolation, so see if that helps you search.

3

Thank you, ultrafilter, for some reason I couldn’t think of that term! The function was interp1.

Now I’ve just gotta work out why my data’s messed up.

4

I am not sure how you are using polyval here, but ployfit is what you want to use to fit a set of data. polyfit(x,y,1) will do a linear fit. [polyfit(x,y,2) does a quadratic, polyfit(x,y,3) does a cubic etc. etc.] You will get a vector of two terms from there. Then you do polyval([vector from polyfit],#), where # is the x value that you want to calculate y at.

5

Yes, got them mixed up in my OP, oops.