One dimension missing on hover after rasterize point data

Datashader
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1

Hi, I am trying to make scatter plot for large data with holoviews.operation.datashader.rasterize and show the hover. But I find one dimension of the data is missed on the hover after I use the rasterize. Here is the minimal example code:

import numpy as np
import datashader
import geoviews as gv
gv.extension('bokeh')
from holoviews.operation.datashader import rasterize

lon = np.array([8.75,8.76,8.77])
lat = np.array([46.1,46.2,46.3])
height = np.array([0.0,0.1,0.2])
index = np.array([0,1,2])

points = gv.Points(np.stack((lon,lat,height,index),axis=-1),
                   kdims=['Longitude','Latitude'],vdims=['Height','Index'])
points.opts(tools=['hover'])

The result looks perfect, Longitude, Latitude, Height and Index are all showed. (sorry I can only upload one image as a new user on this website. So, I upload the most important one.)
The I rasterize it:

points = rasterize(points,aggregator=datashader.mean('Height'),vdim_prefix='',precompute=True)
points.opts(tools=['hover'])

image
The Index dimension is missing on the hover.

Then I try to print the rasterized points:

print(points)

and get:

:DynamicMap   []
   :Image   [Longitude,Latitude]   (Height)

It looks like the fourth dimension is missed. I have searched the overall manual but didn’t find anything related to it. Please let me know if anybody knows how to preserve this Index dimension and show it on the hover. Thanks a lot!

1 Like
2

Hi @kanglcn and thanks for taking the time to write down a small code to illustrate what you are after.
The following snippet uses inspect from holoviews.operations.datashader. It is designed to give back the hover functionnality that is otherwise lost with the aggregation that datashader does (it is not much documented for now so you can have a look at Jean-Luc Stevens: Seeing the needle AND the haystack: single-datapoint selection for billion-poin... - YouTube, starting at 13m for some details on inspect, but the whole video is really worth watching).

  • I put you data in a dataframe to make things cleaner and also because I tried to achieve the hover through hvplot.pandas (without success).
  • The dynspread is here to make datashader points more visible, so things are easier for the hover inspection.
  • The code does not work with gvpoints. Indeed, the points disappear as soon as the mouse enters the frame. This looks like a bug.
  • For more inspiration on inspect, have a look at the AIS demo which is extremely good.
import numpy as np
import pandas as pd
import holoviews as hv
#import geoviews as gv
hv.extension('bokeh')
import holoviews.operation.datashader as hd

lon = np.array([8.75,8.76,8.77])
lat = np.array([46.1,46.2,46.3])
height = np.array([0.0,0.1,0.2])
index = np.array([0,1,2])

df = pd.DataFrame(dict(Longitude=lon,Latitude=lat,Height=height,Index=index))

#gvpoints = gv.Points(df, ['Longitude','Latitude'])
hvpoints = hv.Points(df, ['Longitude', 'Latitude'])

rasterized = hd.dynspread(hd.rasterize(hvpoints).opts(width=300, height=300))
highlight= hd.inspect_points(rasterized)
rasterized * highlight.opts(marker='o',size=10,tools=['hover'])
3 Likes
3

@marcbernot thank you for your thoughtful response. I am having a similar problem, I need the extra dimension (in my case an ID attribute from a database) to add actions on selected points.

Your script works, but it does not on my end; the points disappear when I move the mouse cursor in the figure. I guess because I have more points? Not sure. I tried hd.Points, hv.Points, gv.Points and nothing worked. Curious if there have been any developments since your post. Thank you!