Allow re-creating charts on existing canvas that has been scaled before on retina display

When creating new chart on a canvas that has been used before, there's a problem with double-scaling according to the `window.devicePixelRatio` attribute. (Issue #161 mentions this bug.)
This fix enables using the same canvas infinite times, as it assures the device ratio will be saved and only re-calculated when required (mostly during debug/testing sessions when you change the device ratio using the developer tools.)