Fail earlier in case of multiple levelplots

[mvkorpel]

The function plot.mboost() does not allow drawing multiple levelplots. Therefore there is no point in creating and storing (in RET) more than one such plot. I'm suggesting this patch which may save a lot of resources (processing time and memory) by not calling levelplot() more than once.

[coveralls]

Coverage increased (+0.009%) to 79.484% when pulling df73cc7 on mvkorpel:one-levelplot into b893002 on boost-R:master.

[hofnerb]

Good point and valid solution.

However, one could also return all levelplots and subsequently plot the list by hand or by explicitly calling print() on each level plot at the end of the function. Any thoughts?

[mvkorpel]

I don't have a strong opinion on how multiple levelplots should be dealt with. It appears that the current policy of no more than one levelplot was introduced in commit 7e7ef57, release: mboost 2.1-0. I don't know the reasoning behind the change. The objects returned by levelplot() can be large so maybe that's got something to do with the limitation, but the plot objects are stored anyway before throwing the error (before my suggested patch), so there goes that argument... Maybe there could be one or more parameters that let the user decide if (multiple) levelplots should be returned (for later customization or layout adjustments) and/or drawn immediately.