Pixel problem when moving drawings

[IndieKid]

Hey folks, this is a small graph app, you can drag graphs via left-button. When dragging something goes bad with pixels (look at the grid and graphs). I tried to set other Image Filter, but that doesn't seem to be helpful. Grid can be fixed with changing alpha(line 93) from 100 to 255, but I need that to be 100. Can you tell me how to fix that? Thanks!

[Lafolie]

Are you drawing at non-integer co-ordinates when the mouse button is held?

[IndieKid]

Are you drawing at non-integer co-ordinates when the mouse button is held?

Nope.

[davisdude]

If you don't know how to do that, just do something like this:

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love.graphics.draw( 'name', math.floor(name.x), math.floor(name.y))

[IndieKid]

If you don't know how to do that, just do something like this:

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love.graphics.draw( 'name', math.floor(name.x), math.floor(name.y))

Tried before. No result.

[IndieKid]

It seems to be caused by

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boardField:clear()
timeField:clear()

--EDIT
Sure... But without this it will add drawings to canvas, but I need it just to update canvas.
If I put this code in love.update(), it will always draw those wrong pixels. Is there a way to call these functions only if there was something changed from the last tick?