color_levels sometimes picks first n colors, rather than spanning cmap

[jsignell]

I spent a while on hvplot today trying to get the new colormap to work properly for contours. I think the bug is actually in holoviews.

def circle(radius):
    angles = np.linspace(0, 2*np.pi, 100)
    return {'x': radius*np.sin(angles), 'y': radius*np.cos(angles), 'radius': radius}

hv.Contours([circle(i) for i in np.linspace(0, 1, 10)], vdims='radius').opts(cmap='kbc_r', color_levels=5, colorbar=True, width=400)

[jsignell]

This especially matter for hvplot because we intend to switch the default cmap to 'kbc_r' in the next release.