title: pip install spectrum with no extras
body: <p>I pip-installed <code>spectrum</code> which by default installed <code>matplotlib</code>. Looking at the <a href="https://github.com/cokelaer/spectrum/blob/master/setup.py" rel="nofollow noreferrer">source code</a>, I see that <code>matplotlib</code> is in <code>extras_required</code>. I wonder how I can install <code>spectrum</code> without <code>matplotlib</code>.</p>