Enable logarithmic scale in evolution charts

[gka]

If a site experiences exceptional traffic peaks, it would make a lot sense to change the linear y-scale to a logarithmic scale.

For instance, consider the following pattern (data taken from a real website). The linear scale does a good job in giving a sense of the magnitude of the peak, but the patterns in the regular traffic are not perceivable anymore.

[[Image(http://vis4.net/tmp/linear-scale.png)]]

The next picture shows the same data on a log scale, which gives additional insight. We can see how the traffic decreases in two weeks after the peak and then it slightly increases again over the next four weeks.

[[Image(http://vis4.net/tmp/logarithmic-scale.png)]]

I know that this is a feature for pro data analysis, and thus the UI doesn't need to be super flashy. Most users don't need this, so we don't want to put it in their way. I recommend to use shift-clicking on the y-axis labels to toggle between linear and logarithmic scale.

[gka]

Here's another example taken from my website: I had a peak in visitors coming from one region in Germany. When I compare the regions using row evolution, the linear scale makes it impossible to compare the 'regular' traffic between regions. A log-scale would reveal more insights.

[[Image(http://vis4.net/tmp/comparing-regions.png)]]

Again, here's the same data shown in a log-scale:

[[Image(http://vis4.net/tmp/comparing-log.png)]]

[mattab]

Good idea, this could be an option available in the "Cog icon" for evolution graphs. We could also add a link to use the special view when a peak is detected in the graph (eg. more than 0.9 percentile). See: #3110

[halfdan]

This should be relatively easy to implement, since there is a plugin for jqPlot: http://www.jqplot.com/docs/files/plugins/jqplot-logAxisRenderer-js.html

I'll prepare a pull request.