Plugin setting to get logarithmic graphs

[janl]

Currently the plugin has to set graph_args to --logarithmic to get a logarithmic graph. If for some reason the administrator wants to set logarithmic on a graph s/he has to re-specify the whole graph_args on the server. This is clumsy. The --base and -u and -l are very intimate to the graph so it's natural for the plugin to decide. If logarithmic is used should be set by something like graph_scaling linear|logarithmic|auto and then the server does the propper string append to fix this for the plugin.