Minor fixes for legends
Minor fixes for legends

Legends can now be explicitly allowed even for single labels
if show_legend is set to True. Additionally, legends avoid
drawing multiple entries for the same label

ioam/holoviews,ioam/holoviews,mjabri/holoviews,basnijholt/holoviews,mjabri/holoviews,ioam/holoviews,vascotenner/holoviews,mjabri/holoviews,basnijholt/holoviews,vascotenner/holoviews,vascotenner/holoviews,basnijholt/holoviews