The calculation of the horizontal grid does not work when the span is zero. This leads to vertical lines drawn at every point, so the background becomes flat gray and the Smith chart is no longer visible (as reported here: https://groups.io/g/NanoVNA-V2/message/1146).
This pull request sets the grid width to the whole frame in zero span mode, so there is no horizontal grid. This also serves as a visual cue that a time sweep is displayed instead of a frequency sweep.
The calculation of the horizontal grid does not work when the span is zero. This leads to vertical lines drawn at every point, so the background becomes flat gray and the Smith chart is no longer visible (as reported here: https://groups.io/g/NanoVNA-V2/message/1146). This pull request sets the grid width to the whole frame in zero span mode, so there is no horizontal grid. This also serves as a visual cue that a time sweep is displayed instead of a frequency sweep.