Partially fixed; ignoring v-slope during math interval

This makes things more sane, but still does not predict ticks
accurately.  Plot like this:

   gnuplot --persist -e "plot 'exponential-v20000.trace'
      u 2:3 with lines t 'Velocity',
   '' u 2:4 with lines t 'Position (ideal)',
   '' u 2:(floor(\$4+0.5)) with steps t 'Position (ideal int)',
   '' u 2:5 with steps t 'Position (actual)'"

Notice the steps are sometimes long and sometimes short.  Bad predictions?