Certain charts could do with more than 2 metrics. Example: if you compare CTR, Conv. Rate and Impressions - we can successfully display CTR & Conv. Rate in the same axis whilst sending impressions to a secondary axis. But this is not currently possible. My workaround is to create 2 charts and put one on top of the other but this prevents interaction with the chart under the top one.
Allow 2+ metrics in charts
a1: most a2: medium a3: no