Fix errors with recent gnuplot versions

Fix errors with recent gnuplot versions

It looks like recent gnuplot versions do not keep variables around when
they get out of scope, like a counter used only inside a loop.

This results in some errors in the modular scripts:

  ./modular_cartesian.gnuplot", line 62: undefined variable: i
  ./modular_polar.gnuplot", line 59: undefined variable: i

Fix this by using 'year_max' instead of 'i' outside of the loop where
'i' was defined.

Since 'year_max' is a float variable string concatenation would not work
with it, resulting in the following errors:

  ./modular_cartesian.gnuplot", line 62: internal error : STRING operator applied to undefined or non-STRING variable
  ./modular_polar.gnuplot", line 59: internal error : STRING operator applied to undefined or non-STRING variable

So while at it also convert the float to an integer so that the result
can be used with the string concatenation operator.