interseccion en graficos

1) You confuse "ton" (which is mass) with "tonnef" (which is force). Guess you should use "tonnef" throughout

2) Misuse of a range! The function expression with range l as argument are NOT vectors. Actually the aren't ranges, too. These expressions are invalid objects and can only be used in plots and for displaying the values. You can see thats an invalid object if you try to assign it a variable - you will get an error. So you have to create the vectors needed for interpolation by "legal" means instead

3) P -> M is a function. So we can easily determine the (unique!) M value corresponding to a given P value

But M -> P is NOT a function because most M-values correspond to 2 (maybe some to even more) P values. Thats the reason a simple interpolating function does not work. In my approach I used the "root" function and provided a guess range so that only values in the upper 'half' would be returned. The P value of the right hand peak must be determined first - I had chosen 60 tonnef just by optical inspection. Also the end value provided in the "root" function depends on the specific values given and its different in your new example.

Of course it would also be possible to write a function which returns a vector of ALL possible P-values in one go. But it would be some work - especially because some special cases would have to be considered.

Answer in English preferred (or in German as this is my native language 😉 )