Support multiple column levels (#509)

* Support multiple column levels

* Make repeated column titles span multiple columns

* Select table data with iloc rather than loc

* Include header breaks from  header levels

* remove unnecessary listcomp

* remove space from comment

* Go from itertools.accumlate to numpy.cumsum for python 2.7 compatibility

* remove numpy dependency

* Simplify running total

dash_bootstrap_components/_table.py

@@ -1,3 +1,7 @@
+from functools import reduce
+from itertools import groupby
+from operator import add
+
 import dash_html_components as html
 
 
@@ -70,15 +74,69 @@ def _generate_table_from_df(
                 )
         elif isinstance(header, dict):
             df = df.rename(columns=header)
+
+        # Get the actual headers
+        n_levels = df.columns.nlevels
+        header_values = [
+            list(df.columns.get_level_values(level))
+            for level in range(n_levels)
+        ]
+
+        # The sizes of consecutive header groups at each level
+        header_spans = [
+            [len(list(group)) for _, group in groupby(level_values)]
+            for level_values in header_values
+        ]
+
+        # The positions of header changes for each level as an integer
+        header_breaks = [
+            [sum(level_spans[:i]) for i in range(1, len(level_spans) + 1)]
+            for level_spans in header_spans
+        ]
+
+        # Include breaks from higher levels
+        header_breaks = [
+            sorted(set(reduce(add, header_breaks[:level])).union({0}))
+            for level in range(1, n_levels + 1)
+        ]
+
+        # Go from header break positions back to cell spans
+        header_spans = [
+            reversed(
+                [
+                    level_breaks[i] - level_breaks[i - 1]
+                    for i in range(len(level_breaks) - 1, 0, -1)
+                ]
+            )
+            for level_breaks in header_breaks
+        ]
+
         table = [
-            html.Thead(html.Tr(children=[html.Th(col) for col in df.columns]))
+            html.Thead(
+                [
+                    html.Tr(
+                        children=[
+                            html.Th(
+                                header_values[level][pos],
+                                colSpan=span,
+                            )
+                            for pos, span in zip(
+                                header_breaks[level], header_spans[level]
+                            )
+                        ]
+                    )
+                    for level in range(n_levels)
+                ]
+            )
         ]
     else:
         table = []
     table.append(
         html.Tbody(
             [
-                html.Tr([html.Td(df.iloc[i][col]) for col in df.columns])
+                html.Tr(
+                    [html.Td(df.iloc[i, j]) for j in range(len(df.columns))]
+                )
                 for i in range(len(df))
             ]
         )