Fix typo in xIncrement calculation (#131)

• Add output folder to gitignore

• Fix typo in xIncrement calculation

• Update reference plot for failing cross-y chart

• Add test for line chart with small x-range

This should verify that the fix from #131 works.

• Add Quartz-rendered version of testLineChart_smallXRange

• Update reference images

• Disable testLineChart_crossY for now

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