[graph] terminals smaller than 80x25 attempt to draw offscreen

[midichef]

Small description If a window is smaller than the default startup window, canvas.py thinks the screen is bigger than it is, and attempts to draw offscreen at first. This causes errors to show up from various calls to scr.addstr():

scr.addstr(y, x, disp_column_fill*actualw, cattr.attr)  # clear whole area before displaying
_curses.error: addwstr() returned ERR

Steps to reproduce with sample data and a .vd Open a terminal narrower than 80 columns or shorter than 25 columns, and plot any data set.

Additional context saul.pw/VisiData v3.0dev

I'm looking into the cause and a fix.