[canvas-] fix division by zero when col's only value is a large float

[midichef]

When plotting a graph, the plotter tries to prevent xmin == xmax by expanding the range. It adds 1 to xmax in that situation. (We ran into this problem before in #1673.) But that fails for large floats, because adding 1 leaves the number unchanged, as in x=2**53; float(x) == float(x + 1). (Because math.ulp(2**53) is 2.0.)

Example, when plotting the first 2 or last 2 lines of this data with .:

x,y
9007199254740992,1
9007199254740992,2
1,9007199254740992
2,9007199254740992

visidata 3.3 gives ZeroDivisionError: float division by zero because the width or height of the box is zero when xmin == xmax or ymin == ymax:

File "/home/midichef/.venv/lib/python3.12/site-packages/visidata/canvas.py", line 648, in xScaler
    xratio = self.plotviewBox.w/(self.canvasBox.w*self.xzoomlevel)

and

File "/home/midichef/.venv/lib/python3.12/site-packages/visidata/canvas.py", line 657, in yScaler
    yratio = self.plotviewBox.h/(self.canvasBox.h*self.yzoomlevel)

This PR handles the case of such large floats.