@@ -1479,7 +1479,10 @@ def bands(sys, args=(), momenta=65, file=None, show=True, dpi=None,

 def spectrum(syst, x, y=None, params=None, mask=None, file=None,

              show=True, dpi=None, fig_size=None, ax=None):

-    """Plot the spectrum of a Hamiltonian as a function of 1 or 2 parameters

+    """Plot the spectrum of a Hamiltonian as a function of 1 or 2 parameters.

+

+    This function requires either matplotlib or plotly to be installed.

+    The default backend uses matplotlib for plotting.

     Parameters

     ----------

@@ -1500,32 +1503,67 @@ def spectrum(syst, x, y=None, params=None, mask=None, file=None,

         if the spectrum should not be calculated for the given parameter

         values.

     file : string or file object or `None`

-        The output file.  If `None`, output will be shown instead.

+        The output file.  If `None`, output will be shown instead. If plotly is

+        selected as the backend, the filename has to end with a html extension.

     show : bool

-        Whether ``matplotlib.pyplot.show()`` is to be called, and the output is

-        to be shown immediately.  Defaults to `True`.

+        For matplotlib backend, whether ``matplotlib.pyplot.show()`` is to be

+        called, and the output is to be shown immediately.

+        For the plotly backend, a call to ``iplot(fig)`` is made if

+        show is True.

+        Defaults to `True` for both backends.

     dpi : float

         Number of pixels per inch.  If not set the ``matplotlib`` default is

         used.

+        Only for matplotlib backend. If the plotly backend is selected and

+        this argument is not None, then a RuntimeError will be triggered.

     fig_size : tuple

         Figure size `(width, height)` in inches.  If not set, the default

         ``matplotlib`` value is used.

+        Only for matplotlib backend. If the plotly backend is selected and

+        this argument is not None, then a RuntimeError will be triggered.

     ax : ``matplotlib.axes.Axes`` instance or `None`

         If `ax` is not `None`, no new figure is created, but the plot is done

         within the existing Axes `ax`. in this case, `file`, `show`, `dpi`

         and `fig_size` are ignored.

+        Only for matplotlib backend. If the plotly backend is selected and

+        this argument is not None, then a RuntimeError will be triggered.

     Returns

     -------

-    fig : matplotlib figure

-        A figure with the output if `ax` is not set, else None.

+    fig : matplotlib figure or plotly Figure object

     """

-    if not _p.mpl_available:

-        raise RuntimeError("matplotlib was not found, but is required "

-                           "for plot_spectrum()")

-    if y is not None and not _p.has3d:

-        raise RuntimeError("Installed matplotlib does not support 3d plotting")

+    params = params or dict()

+

+    if get_backend() == _p.Backends.matplotlib:

+        return _spectrum_matplotlib(syst, x, y, params, mask, file,

+                                    show, dpi, fig_size, ax)

+    elif get_backend() == _p.Backends.plotly:

+        if(dpi or fig_size or ax):

+            raise RuntimeError('Incompatible arguments of dpi, fig_size, or '

+                               'ax. Current plotting backend is plotly.')

+        return _spectrum_plotly(syst, x, y, params, mask, file, show)

+

+

+def _generate_spectrum(syst, params, mask, x, y):

+    """Generates the spectrum dataset for the internal plotting

+    functions of spectrum().

+

+    Parameters

+    ----------

+    See spectrum(...) documentation.

+

+    Returns

+    -------

+    spectrum : Numpy array

+         The energies of the system calculated at each coordinate.

+    planar : bool

+         True if y is None

+    array_values : tuple

+         The coordinates of x, y values of the dataset for plotting.

+    keys : tuple

+         Labels for the x and y axes.

+    """

     if system.is_finite(syst):

         def ham(**kwargs):

@@ -1536,9 +1574,9 @@ def spectrum(syst, x, y=None, params=None, mask=None, file=None,

         raise TypeError("Expected 'syst' to be a finite Kwant system "

                         "or a function.")

-    params = params or dict()

-    keys = (x[0],) if y is None else (x[0], y[0])

-    array_values = (x[1],) if y is None else (x[1], y[1])

+    planar = y is None

+    keys = (x[0],) if planar else (x[0], y[0])

+    array_values = (x[1],) if planar else (x[1], y[1])

     # calculate spectrum on the grid of points

     spectrum = []

@@ -1558,10 +1596,91 @@ def spectrum(syst, x, y=None, params=None, mask=None, file=None,

     new_shape = [len(v) for v in array_values] + [-1]

     spectrum = np.array(spectrum).reshape(new_shape)

+    return spectrum, planar, array_values, keys

+

+

+def _spectrum_plotly(syst, x, y=None, params=None, mask=None,

+                     file=None, show=True):

+    """Plot the spectrum of a Hamiltonian as a function of 1 or 2 parameters

+    using the plotly backend.

+

+    Parameters

+    ----------

+    See spectrum(...) documentation.

+

+    Returns

+    -------

+    fig : plotly Figure / dict

+    """

+

+    if not _p.plotly_available:

+        raise RuntimeError("plotly was not found, but is required for using"

+                           "the spectrum() plotly backend")

+

+    spectrum, planar, array_values, keys = _generate_spectrum(syst, params,

+                                                              mask, x, y)

+

+    if planar:

+        fig = _p.plotly_graph_objs.Figure(data=[

+          _p.plotly_graph_objs.Scatter(

+                 x=array_values[0],

+                 y=energies,

+          ) for energies in spectrum.T

+        ])

+        fig.layout.xaxis.title = keys[0]

+        fig.layout.yaxis.title = 'Energy'

+        fig.layout.showlegend = False

+    else:

+        fig = _p.plotly_graph_objs.Figure(data=[

+          _p.plotly_graph_objs.Surface(

+                 x=array_values[0],

+                 y=array_values[1],

+                 z=energies,

+                 cmax=np.max(spectrum),

+                 cmin=np.min(spectrum),

+          ) for energies in spectrum.T

+        ])

+        fig.layout.scene.xaxis.title = keys[0]

+        fig.layout.scene.yaxis.title = keys[1]

+        fig.layout.scene.zaxis.title = 'Energy'

+

+    fig.layout.title = (

+        ', '.join('{} = {}'.format(*kv) for kv in params.items())

+    )

+

+    _maybe_output_fig(fig, file=file, show=show)

+

+    return fig

+

+

+def _spectrum_matplotlib(syst, x, y=None, params=None, mask=None, file=None,

+                         show=True, dpi=None, fig_size=None, ax=None):

+    """Plot the spectrum of a Hamiltonian as a function of 1 or 2 parameters

+    using the matplotlib backend.

+

+    Parameters

+    ----------

+    See spectrum(...) documentation.

+

+    Returns

+    -------

+    fig : matplotlib figure

+        A figure with the output if `ax` is not set, else None.

+    """

+

+    if not _p.mpl_available:

+        raise RuntimeError("matplotlib was not found, but is required for"

+                           "using the spectrum() matplotlib backend")

+    if y is not None and not _p.has3d:

+        raise RuntimeError("Installed matplotlib does not support 3d plotting")

+

+    spectrum, planar, array_values, keys = _generate_spectrum(syst, params,

+                                                              mask, x, y)

+

     # set up axes

     if ax is None:

         fig = _make_figure(dpi, fig_size, use_pyplot=(file is None))

-        if y is None:

+        if planar:

             ax = fig.add_subplot(1, 1, 1)

         else:

             warnings.filterwarnings('ignore',

@@ -1569,7 +1688,7 @@ def spectrum(syst, x, y=None, params=None, mask=None, file=None,

             ax = fig.add_subplot(1, 1, 1, projection='3d')

             warnings.resetwarnings()

         ax.set_xlabel(keys[0])

-        if y is None:

+        if planar:

             ax.set_ylabel('Energy')

         else:

             ax.set_ylabel(keys[1])

@@ -1585,7 +1704,7 @@ def spectrum(syst, x, y=None, params=None, mask=None, file=None,

         fig = None

     # actually do the plot

-    if y is None:

+    if planar:

         ax.plot(array_values[0], spectrum)

     else:

         if not hasattr(ax, 'plot_surface'):