Here's an extract from the Advanced configuration - Layout page..
For example, say you want to define two charts where one overlays the other so that they share the same x-axis, but have different y-axes (one on the left side of the chart, and another on the right side of the chart). You would start by defining two custom chart types, chart1 and chart2. We can configure the charts list to render those custom charts instead of the default chart:
[@chart1,@chart2]
More information about configuring multiple charts that share axes is coming soon
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