[FR] Show legend by default only if labels have been set

[mgkuhn]

At the moment the default is to always show a legend, i.e.legend=true, even if no label was provided by the user. The default label "y1" is not really useful, especially not if there is just a single series. As a result, users have to add legend=false very frequently.

For v2 it would be nice if the legend is only shown by default if label values have been provided, especially if there is only a single series.

Current result:
plot(sin) looks like plot(sin; legend=true, label=("y1"))

Expected result:
plot(sin) should look like plot(sin; legend=false)
plot(sin; label=("sin")) should look like plot(sin; legend=true, label=("sin"))