update playground and fourier plots to work with bokeh 2

Author: surfaceowl

notebooks/02_bokeh_chapter.ipynb

@@ -1927,7 +1927,7 @@
     "from bokeh.layouts import grid\n",
     "from bokeh.models import CustomJS, ColumnDataSource\n",
     "from bokeh.models import Button  # for saving data\n",
-    "from bokeh.events import ButtonClick\n",
+    "from bokeh.events import ButtonClick  # for saving data\n",
     "from bokeh.models.widgets import DataTable, DateFormatter, TableColumn\n",
     "from bokeh.models import HoverTool\n",
     "from bokeh.plotting import figure\n"