Although this is not my field, I think I can help you.
a) Your graph should have two vertical lines, of equal height at mass 13 and 15 respectively.
b) Only 1 isotope would be present, i.e. the principal element with mass M=13 and then one isotope at mass M+2
We are assuming that the principal element is the one that is the lowest mass - by definition, an isotope is one where there are additional neutrons - hence the mass increases, but the proton count is the same. Having an equal height means that the relative abundance of both are the same.
