[FEATURE] hist.plot legend automatically

[andrzejnovak]

Currently running

h = hist.new.Reg(100, 0, 10, label="X").StrCat([], label="Sample", growth=True).Weight()\
        .fill(np.random.normal(5, 1, int(1e5)), np.random.choice(list("ABCDEF"), int(1e5)))
h.plot1d(stack=True, histtype='fill', sort='label');
plt.legend()
hep.cms.label();

doesn't automatically plot the legend, meaning that when user triggers this themselves the "Sample" label won't appear automatically because there is nowhere to store it within the plot1d method without calling legend.

Should we include the legend automatically so that the legend title can be read from the axis info? The downside the user having slightly worse access to the legend object compared to they themselves running leg = plt.legend()