FIX/ENH: attempt soft conversion of object series before raising a TypeError when plotting
this leads the user to believe that `object` types are ok; I think you previous solution (of raising) is better as it has a failure that is more obvious

@wesm ?
 i like the raise too since if you have plottable series that are not numeric you should convert, but @wesm gave the raise-on-object a strong :-1: in #3911 .
 oh I see

ok maybe do the conversion if needed (and warn if u can do the conversion but let it proceed - PerformanceWarning)
and raise like u do now (if u can't convert it)
 ok i'll add the perf warning.
 @jreback ok 2 merge?
 @jreback any objections? going 2 merge if not...
 No please do not put in a performance warning! Thank you for fixing this though
 sure thing. hope i didn't ruffle too many feathers. trying to avoid :-1:s like the plague! almost finished
 @wesm merge-able?
 @jreback merge? anyone, beuller? trying to assist in as many 0.11.1 issues as i can...
 ok then
