ILinePlot: preserve order in the legend of ILinePlot

In aa4bece3 ("ILinePlot: don't call data_frame what's just a python dict") we turned a pd.Series to a dict, because we were effectively using it as a dict. However, there was a behaviour change as a result of this: pandas series preserve the order of entries whereas python dictionaries may reorder its entries. Use an OrderedDict instead, as that's what aa4bece3 should have done.