Plotting - Improve the custom labelling capability

[Davide-sd]

Whenever two expressions are being plotted with the same plot command, a string representation of the expression is automatically generated:

expr1 = (cos(x) + sin(x * pi / 4)) * exp(-x**2 / 20)
expr2 = cos(x)
p = plot(expr1, expr2, legend=True)

For long expression, this is quite inconvenient as the legend will eventually cover the plot. At the moment, it is possible to change the label with the following:

p._series[0].label = "a"
p._series[1].label = "b"
p.show()

However, this is unnecessary complicated as it requires two steps/multiple commands to get a single plot. Ideally, something similar to the following would be much appreciated:

plot((expr1, "a"), (expr2, "b"), legend=True)