You should be able to view, for example, Page Views and Visitors in the same table and chart. One uses the left axis, the other the right. It lets you see any correlation.
Isn't this the same as my Trend 2 metrics idea?
Yeah. I had posted it first on the beta site before you, but someone moved it here for me the other day. It's slightly different in that it focuses on the charting, but essentially the same request.
Here is a prototype double y axis chart. Is this what you have in mind? Another option based on adamgreco's suggestion would be to have multiple metrics trended on one chart but on the same scale. Also, the hybrid would be to have 2 y axis and have multiple metrics and choose which scale they will be plotted on. These are just all ideas. What are your thoughts on this?
That looks promising. I think the key is to visually tie the lines to the axis to avoid confusion.Example:
The colors reinforce the difference in scale. Another option is to use columns vs. lines.
For charting multiple series per axis, you could use either color shades -- dark to light of color x for axis 1, different color shades for axis 2. Yet another option for multiple lines is to use same color but dotted, dashed variations to differentiate values.