Piecewise functions and legend label

edit

This is a bug, I think. Here is the documentation for Piecewise plotting.

def plot(self, *args, **kwds):
    """
    Returns the plot of self.

    Keyword arguments are passed onto the plot command for each piece
    of the function. E.g., the plot_points keyword affects each
    segment of the plot.

So that includes the legend ones, which however, is not what you want, as opposed to some of the other options. I've added this to the piecewise plotting bug ticket.

Update: Here is a workaround of sorts. Unfortunately, it means plotting each part separately, only labeling one of them.

sage: P = plot(Piecewise([[(0,1),x]]),legend_label='f(x)')
sage: Q = plot(Piecewise([[(1,2),x^2]]),legend_label='')
sage: R = plot(Piecewise([[(0,1),x^2]]),legend_label='g(x)',color='red')
sage: S = plot(Piecewise([[(1,2),x]]),legend_label='',color='red')
sage: P+Q+R+S

Or you could just skip the legend_label option totally on Q and S.

Here's a slightly different (banana-themed) workaround. Run the following before making the plot:

# monkeypatch legend duplication of trac #11225
def fix_piecewise(fn):
    import types
    def fixed_plot(self, *args, **kwargs): 
        from sage.plot.all import plot
        return sum([plot(f, a, b, *args, **(dict((k,v) for k,v in kwargs.items() if i == 0 or k != 'legend_label')))
                    for i, ((a,b),f) in enumerate(self.list())])
    def fn2(*args, **kwargs):
        ans = fn(*args, **kwargs)
        ans.plot = types.MethodType(fixed_plot, ans)
        return ans
    return fn2

Piecewise = fix_piecewise(Piecewise)

after which it should behave.