# Dynamic Map with TapSelection

**URL:** <https://discourse.holoviz.org/t/dynamic-map-with-tapselection/4426>\
**Category:** HoloViews\
**Tags:** datashader\
**Created:** [October 21, 2022, 7:41pm UTC](https://discourse.holoviz.org/t/dynamic-map-with-tapselection/4426 "2022-10-21T19:41:20Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![LorenzoP](https://discourse.holoviz.org/user_avatar/discourse.holoviz.org/lorenzop/32/4142_2.png) [@LorenzoP](https://discourse.holoviz.org/u/LorenzoP)\
**Post date:** [October 21, 2022, 7:41pm UTC](https://discourse.holoviz.org/t/dynamic-map-with-tapselection/4426/1 "2022-10-21T19:41:21Z")

</div>

Hello,

What I am trying to do is allow the user to tap on the map and plot time series data for the n-closest points.

The tap stream along with the Dynamic map work for the rasterized image. The problem is if I use an `hv.overlay` (that is the commented part). The reason for using an `hv.overlay` is that I will create a list of `hv.Curves` with a time series for each and then I can use `hv.Overlay(list_of_curves)`

As an alternative, Can I make the callable function `clicked_point` return a Bokeh plot?

Thanks

```auto
from pyproj import Transformer
import holoviews as hv
import pandas as pd
import numpy as np
import holoviews.operation.datashader as hd
import datashader as ds
from holoviews import opts

hv.extension("bokeh")
renderer = hv.renderer("bokeh").instance(mode="server")
TOOLS = ['box_select', 'lasso_select', 'reset', 'tap']

# Dummy data
lats = np.linspace(30.266,30.466, num=1000)
lons = np.linspace(-97.73,-97.93, num=1000)
velocity = np.random.randint(0, 10000, size=1000)
df = pd.DataFrame({'latitude': lats, 'longitude': lons, 'velocity': velocity})

# Transform to mercator
lonlat_to_webmercator = Transformer.from_crs(
    "EPSG:4326", "EPSG:3857", always_xy=True)

df['longitude_mercator'], df['latitude_mercator'] = \
    lonlat_to_webmercator.transform(
        df['longitude'], df['latitude'])

map_points = hv.Points(
    df,
    ["longitude_mercator", "latitude_mercator"]
    )

spreaded = hd.dynspread(
        hd.rasterize(
            map_points,
            aggregator=ds.mean('velocity'),
            element_type=hv.Image,
            precompute=True,
        ),
        threshold=0.5,
        max_px=10
    ).opts(tools = ['hover'])

map_plot = spreaded.opts(
    opts.Image(
        cmap="rainbow_r",
        cnorm='linear',
        colorbar=True,
        clabel="mm/yr",
        bgcolor="black",
        height=700,
        toolbar="left",
        responsive=True,
        show_frame=False,
        xaxis=None,
        yaxis=None,       
    ),
)

extracted_dims = hd.inspect_points(map_plot)
map_plot = hv.element.tiles.EsriImagery() * map_plot \
            * extracted_dims.opts(tools=['hover'] + TOOLS)

posxy = hv.streams.Tap(source=spreaded, x=0, y=0)

# Doesnt work
# def clicked_point(x, y):
# if x == 0 and y == 0:
# return hv.Overlay([hv.Points([])])
# return hv.Overlay([hv.Points(data={'x': [x,x+50], 'y': [y, y+50]})])

def clicked_point(x, y):
    if x == 0 and y == 0:
        return hv.Points([])
    return hv.Points(data={'x': [x,x+50], 'y': [y, y+50]}).opts(marker = 'triangle', size = 20 )

tap_dmap = hv.DynamicMap(clicked_point, streams=[posxy]).opts(
             height = 300, 
            responsive=True,
             framewise=True,
         ).opts(tools= TOOLS)

h = hv.Layout(map_plot + tap_dmap).cols(1)
h

```

---

<div class="post-metadata">

**Author:** ![carl](https://discourse.holoviz.org/user_avatar/discourse.holoviz.org/carl/32/5215_2.png) [@carl](https://discourse.holoviz.org/u/carl)\
**Post date:** [October 26, 2022, 6:46pm UTC](https://discourse.holoviz.org/t/dynamic-map-with-tapselection/4426/2 "2022-10-26T18:46:45Z")

</div>

Hi @LorenzoP,

Is it possibly because overlay does not like being empty, if instead of trying to return an overlay I attempted to create it from hv.Overlay(tap\_dmap) it throws an error vs silently failing not working with the commented out section. Not something I know of how to fix though other than maybe not allowing xy not to be zero never got far enough to test that if possible to see if indeed it is failing because of being empty

The error says something like overlay is empty skipping plot