it seems more natural to stop autoscroll with button 3 and start it
with button 1

not very clean, might be reworked at some point
the idea is to compute much less 'log' than what
was done

not very efficient, to be refined

requires those optimizations

is zero in get_dimensions for xy_plot

this calls clock_gettime for each active T macro, hopefully it
does not eat too much CPU

can be deactivated by commenting one line in T_defs.h