@@ -79,6 +79,17 @@ def set_backend(backend):

 def get_backend():

     return _p.backend

+

+def _check_incompatible_args_plotly(dpi, fig_size, ax):

+    assert(get_backend() == _p.Backends.plotly)

+    if(dpi or fig_size or ax):

+        raise RuntimeError(

+            "Plotly backend does not support setting 'dpi', 'fig_size' "

+            "or 'ax', either leave these parameters unspecified, or "

+            "select the matplotlib backend with"

+            "'kwant.plotter.select_backend(\"matplotlib\")'")

+

+

 def _sample_array(array, n_samples, rng=None):

     rng = _common.ensure_rng(rng)

     la = len(array)

@@ -1539,10 +1550,10 @@ def spectrum(syst, x, y=None, params=None, mask=None, file=None,

         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.')

+        _check_incompatible_args_plotly(dpi, fig_size, ax)

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

+    else:

+        raise RuntimeError("Backend not supported by spectrum().")

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

@@ -45,7 +45,7 @@ def test_matplotlib_backend_unset():

     assert matplotlib_backend_chosen is False

-def test_importable_without_matplotlib():

+def test_importable_without_backends():

     prefix, sep, suffix = _plotter.__file__.rpartition('.')

     if suffix in ['pyc', 'pyo']:

         suffix = 'py'

@@ -55,13 +55,16 @@ def test_importable_without_matplotlib():

         code = f.read()

     code = code.replace(b'from . import', b'from kwant import')

     code = code.replace(b'matplotlib', b'totalblimp')

+    code = code.replace(b'plotly', b'plylot')

     with warnings.catch_warnings(record=True) as w:

         warnings.simplefilter("always")

         exec(code)               # Trigger the warning.

-        assert len(w) == 1

+        assert len(w) == 2

         assert issubclass(w[0].category, RuntimeWarning)

-        assert "only iterator-providing functions" in str(w[0].message)

+        assert issubclass(w[1].category, RuntimeWarning)

+        assert "totalblimp is not available" in str(w[0].message)

+        assert "plylot is not available" in str(w[1].message)

 def syst_2d(W=3, r1=3, r2=8):