Added nytaxi_hover.py to examples/apps/bokeh

examples/apps/bokeh/nytaxi_hover.py

@@ -0,0 +1,52 @@
+"""
+Bokeh app example using datashader for rasterizing a large dataset and
+geoviews for reprojecting coordinate systems.
+
+This example requires the 1.7GB nyc_taxi.csv dataset which you can
+obtain by following the instructions for 'nyc_taxi' at:
+
+  https://github.com/bokeh/datashader/blob/master/examples/README.md
+
+Once this CSV is placed in a data/ subfolder, you can run this app with:
+
+  bokeh serve --show nytaxi_hover.py
+
+"""
+import holoviews as hv
+import geoviews as gv
+import dask.dataframe as dd
+import cartopy.crs as ccrs
+
+from holoviews.operation.datashader import datashade, aggregate
+
+hv.extension('bokeh')
+
+# Set plot and style options
+hv.util.opts('Image [width=800 height=400 shared_axes=False logz=True] {+axiswise} ')
+hv.util.opts("HLine VLine (color='white' line_width=1) Layout [shared_axes=False] ")
+hv.util.opts("Curve [xaxis=None yaxis=None show_grid=False, show_frame=False] (color='orangered') {+framewise}")
+
+# Read the CSV file
+df = dd.read_csv('data/nyc_taxi.csv',usecols=['pickup_x', 'pickup_y'])
+df = df.persist()
+
+# Reproject points from Mercator to PlateCarree (latitude/longitude)
+points = gv.Points(df, kdims=['pickup_x', 'pickup_y'], vdims=[], crs=ccrs.GOOGLE_MERCATOR)
+projected = gv.operation.project_points(points, projection=ccrs.PlateCarree())
+projected = projected.redim(pickup_x='lon', pickup_y='lat')
+
+# Use datashader to rasterize and linked streams for interactivity
+agg = aggregate(projected, link_inputs=True, x_sampling=0.0001, y_sampling=0.0001)
+pointerx = hv.streams.PointerX(x=-74, source=projected)
+pointery = hv.streams.PointerY(y=40.8,  source=projected)
+vline = hv.DynamicMap(lambda x: hv.VLine(x), streams=[pointerx])
+hline = hv.DynamicMap(lambda y: hv.HLine(y), streams=[pointery])
+
+sampled = hv.util.Dynamic(agg, operation=lambda obj, x: obj.sample(lon=x),
+                          streams=[pointerx], link_inputs=False)
+
+hvobj = ((agg * hline * vline) << sampled.opts(plot={'Curve': dict(width=100)}))
+
+# Obtain Bokeh document and set the title
+doc = hv.renderer('bokeh').server_doc(hvobj)
+doc.title = 'NYC Taxi Crosshair'