If you would make a three bar graph, with one of the outer bars the temperature (and/or barometric pressure) and the other the humidity (in % water vapour content), you could place the middle bar such as to reflect the mixture strength or needle setting. With a bit of juggling of the scales, it would even be quite accurate.
You would also need extra tables to retrieve water content from temperature and dew point though. Cold humid air contains very little moisture. At elevated temperatures the ability to contain moisture increases tremendously, until at 200F the moisture content can be 100%, eliminating all air content.