Toggle between linear and logarithmic y-axis on charts

This is important for two use cases:

1. Big spikes in charts are inevitable.  If you are unlucky enough to have a big spike in the middle of a time period you want to visualize, the chart is likely not very useful with a linear y-axis because it becomes dominated by the single spike.
2. Many charts display metrics with vastly different orders of magnitude.  For example, the default PostgreSQL BGWriter Buffers chart, which simultaneously plots metrics with values like 10 and 100,000 on the same chart.  The small values are just a single pixel across the bottom.