Add plt.show() to all plotting calls

This doesn't change current behavior at all. However, without it the
plot gets displayed multiple times when using IPython's widgets as I'm
discovering now.

I've tested this by calling each plot method and verifying that the
notebook is the same. In addition, I've verified that using an IPython
widget does not duplicate plots when redrawn.

datascience/tables.py

@@ -2276,6 +2276,7 @@ def _visualize(self, x_label, y_labels, ticks, overlay, draw, annotate, width=6,
                 if ticks is not None:
                     annotate(axis, ticks)
                 type(self).plots.append(axis)
+        plt.show()
 
     def _split_column_and_labels(self, column_or_label):
         """Return the specified column and labels of other columns."""
@@ -2313,6 +2314,7 @@ def pivot_hist(self, pivot_column_label, value_column_label, overlay=True, width
         t['end'] = bins[1:]
         for label, column in zip(pvt_labels,vals):
             t[label] = column
+        plt.show()
 
     def hist(self, *columns, overlay=True, bins=None, bin_column=None, unit=None, counts=None, width=6, height=4, **vargs):
         """Plots one histogram for each column in columns. If no column is
@@ -2440,6 +2442,7 @@ def hist(self, *columns, overlay=True, bins=None, bin_column=None, unit=None, co
                 axis.hist(values, color=color, **vargs)
                 _vertical_x(axis)
                 type(self).plots.append(axis)
+        plt.show()
 
     def boxplot(self, **vargs):
         """Plots a boxplot for the table.
@@ -2487,6 +2490,7 @@ def boxplot(self, **vargs):
         vargs['labels'] = columns.keys()
         values = list(columns.values())
         plt.boxplot(values, **vargs)
+        plt.show()
 
 
     ###########