Step values don't align properly with logBase

[obfuscurity]

Example graph using the following query:

/render/?width=800&height=400
&logBase=10
&minorY=10
&target=graphite-1.memory.memory-used.value&target=graphite-1.memory.memory-free.value

First reported in https://answers.launchpad.net/graphite/+question/254056.

[stale]

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