More explicit handling of absence of data to plot.

libhts/libhts.py

@@ -409,7 +409,14 @@ def plot_histo(outfile, data, title=None):
     plt.setp(ax.get_xticklabels(), rotation=90)
     if title is not None:
         plt.title(title)
-    plt.savefig(outfile)
+    ## debug
+    try:
+        plt.savefig(outfile)
+    except RuntimeError as e:
+        print(data.index)
+        print(title)
+        raise
+    ##
 
 
 def plot_boxplots(data, ylabel):
@@ -597,6 +604,8 @@ def plot_MA(res,
     """*fold_type* is "log2FoldChange" by default.
     It can also be "lfcMLE", which is based on uncorrected values.
     This may not be good for genes with low expression levels."""
+    if not len(res):
+        raise ValueError("No data to plot.")
     fig, ax = plt.subplots()
     # Make a column indicating whether the gene is DE or NS
     data = res.assign(is_DE=res.apply(set_de_status, axis=1))
@@ -684,6 +693,8 @@ def plot_scatter(data,
                  x_range=None,
                  y_range=None,
                  axes_style=None):
+    if not len(data):
+        raise ValueError("No data to plot.")
     fig, ax = plt.subplots()
     # ax.set_adjustable('box')
     # First plot the data in grey