#1827 plot has incorrect x extent when plotting dynamic data 'with boxes'; replot fixes it

[dima]

Hi. This is similar-to-but-opposite from bug

https://sourceforge.net/p/gnuplot/bugs/1826/

In that entry, a histogram's extents (y specifically) were correct on a plot, but incorrect on a replot. Here they're (x extents this time) incorrect on a plot, but are fixed on a replot. I'm attaching a demo script.