Fitting to a log log plot (Photon Transfer Curves)

[Jbryant61]

Hi, I am trying to fit a straight line to data that is displayed on a log log plot. I can't quite get the straight line to work and assume I am messing up elementary logs. Can anyone give me a pointer to my mistake please and the best way to proceed.

thanks

Jason

[AlanStevens · Accepted solution]

You need to plot like against like! See attached

Alan

[Jbryant61]

Thanks Alan. I like to use the 2nd of your plots (keeping the axis as logs), but then I run into the problem of not being able to find the intercept on the x-axis which is my target.

[AlanStevens]

Your intercept, c, is negative. Mathcad will only plot positive values on a log scale.

Or do you mean the value of x when y is zero (which is just -c/m)? Mathcad still won't plot this on a log-log plot because it won't go down to zero, the values must be positive). Oops, no! That's not right. When y is zero, 10^y is one, so MathCad can show this intercept - see attached.

Alan