In version 0.1.2 the circles don't disappear after the tape measure tool has been closed.

I believe it's due to a typo in the remove_circle function:

local function remove_circle()
    if global.pixesl and global.pixels.circle then -- <~~~~ on this line, s/pixesl/pixels/
        rendering.destroy(global.pixels.circle)
        global.pixels.circle = nil
    end
end

This typo doesn't appear in your main GitHub branch but it's present in the release on mods.factorio.com.

I've tested this locally by fixing the typo and it repairs the bug on my box.

Thanks for this report! Indeed I encountered the same behavior but didn't dig into it yet. Fixed the typo and uploaded a new version.