How to do logarithm y-axis? Using countOver()?

I want to plot a log y axis on a vertical bar chart, e.g. let's imagine I want to show total sales on the y-axis, with customerID on the x-axis. There's no "log" option when you format the visualisation, so I thought that this could be achieved using a calculated field, using log(countOver()). I think I need the POST_AGG_FILTER calculation level as I want the log() calculation to be applied after my data has been grouped by customerID...but this gives me an error when rendering the visualisation of "Table calculation attribute references(s) are missing...".
Anyway - what's the best strategy to achieve a log(<count of something>) on the y-axis?