Problem

Sometimes plots (especially histograms with narrow bins) have small features that are obscured or missed by ordinary dashed line plots.

Proposed Solution

Add the ability to specify an offcolor=(0.0, 0.0, 0.0, 0.0) kwarg that fills in the blanks when a dashed line style, or dash pattern, is used. In principle, even more colors could be specified, but a simple on-color/off-color would be sufficient and require the fewest changes to the API.
This can already be done by plotting multiple lines that have carefully matching dash patterns, but the issue is that the stripes aren't represented in plot legends.

Additional context and prior art

Right now I do this with this function (where the off-color is set to half the opacity of the oncolor):

def plotstripedline(ax, xvals, yvals, oncolor, dashpattern, lname):
    if len(oncolor) != 4:
        raise ValueError(
            "oncolor must have four floating point elements 0 to 1, rgba.")
    linestyle = (0, dashpattern)
    ax.plot(xvals, yvals, ls=linestyle, color=oncolor,
            label=lname)
    
    gapdp = list(dashpattern[1:])
    gapdp.append(dashpattern[0])
    gapstyle = (-dashpattern[0], gapdp)
    gapcol = [ x for x in oncolor ]
    gapcol[-1] *= 0.5
    ax.plot(xvals, yvals, ls=gapstyle, color=gapcol)
    
    return None

See, also, this discussion on StackExchange.