Advanced Dashboards#
At this point we have learned how to quickly build visualizations with hvPlot, how to build interactive apps and dashboards with Panel, and how to add custom interactivity by using HoloViews. In this section we will work on putting all of this together to build complex and efficient data processing pipelines, controlled by Panel widgets.
import pathlib
import colorcet as cc
import pandas as pd
import holoviews as hv
import numpy as np
import panel as pn
import xarray as xr
import hvplot.pandas # noqa: API import
import hvplot.xarray # noqa: API import
pn.extension()
Before we get started let’s once again load the earthquake and population data and define the basic plots, which we will build the dashboard around.
%%time
df = pd.read_parquet(pathlib.Path('../data/earthquakes-projected.parq'))
most_severe = df[df.mag >= 7]
ds = xr.open_dataarray(pathlib.Path('../data/raster/gpw_v4_population_density_rev11_2010_2pt5_min.nc'))
cleaned_ds = ds.where(ds.values != ds.nodatavals).sel(band=1)
cleaned_ds.name = 'population'
mag_cmap = cc.CET_L4[::-1]
high_mag_points = most_severe.hvplot.points(
x='longitude', y='latitude', c='mag', hover_cols=['place', 'time'],
cmap=mag_cmap, tools=['tap'], selection_line_color='black')
rasterized_pop = cleaned_ds.hvplot.image(
rasterize=True, cmap='kbc', logz=True, clim=(1, np.nan),
height=500, width=833, xaxis=None, yaxis=None).opts(bgcolor='black')
CPU times: user 1.64 s, sys: 201 ms, total: 1.84 s
Wall time: 1.37 s
Building Pipelines#
In the previous sections we built a little function to cache the closest earthquakes since the computation can take a little while. An alternative to this approach is to start building a pipeline in HoloViews to do this very thing. Instead of writing a function that operates directly on the data, we rewrite the function to accept a Dataset and the index. This function again filters the closest earthquakes within the region and returns a new Dataset:
from holoviews.streams import Selection1D
def earthquakes_around_point(ds, index, degrees_dist=0.5):
if not index:
return ds.iloc[[]]
row = high_mag_points.data.iloc[index[0]]
half_dist = degrees_dist / 2.0
df = ds.data
nearest = df[((df['latitude'] - row.latitude).abs() < half_dist)
& ((df['longitude'] - row.longitude).abs() < half_dist)]
return hv.Dataset(nearest)
Now we declare a HoloViews Dataset, an Selection1D stream and use the apply method to apply the function to the dataset. The most important part is that we can now provide the selection stream’s index parameter to this apply method. This sets up a pipeline which filters the Dataset based on the current index:
dataset = hv.Dataset(df)
index_stream = Selection1D(source=high_mag_points, index=[-3])
filtered_ds = dataset.apply(earthquakes_around_point, index=index_stream.param.index)
The filtered Dataset object itself doesn’t actually display anything, but it provides an intermediate pipeline stage which will feed the actual visualizations. The next step therefore is to extend this pipeline to build the visualizations from this filtered dataset. For this purpose we define some functions which take the dataset as input and then generate a plot:
hv.opts.defaults(
hv.opts.Histogram(toolbar=None),
hv.opts.Scatter(toolbar=None)
)
def histogram(ds):
return ds.data.hvplot.hist(y='mag', bin_range=(0, 10), bins=20, color='red', width=400, height=250)
def scatter(ds):
return ds.data.hvplot.scatter('time', 'mag', color='green', width=400, height=250, padding=0.1)
# We also redefine the VLine
def vline_callback(index):
if not index:
return hv.VLine(0)
row = most_severe.iloc[index[0]]
return hv.VLine(row.name).opts(line_width=1, color='black')
temporal_vline = hv.DynamicMap(vline_callback, streams=[index_stream])
dynamic_scatter = filtered_ds.apply(scatter)
dynamic_histogram = filtered_ds.apply(histogram)
Now that we have defined our visualizations using lazily evaluated pipelines we can start looking at them. This time we will use Panel to lay out the plots:
pn.Column(
rasterized_pop * high_mag_points,
pn.Row(
dynamic_scatter * temporal_vline,
dynamic_histogram))
Exercise#
Define another function like the histogram or scatter function and then apply it to the filtered_ds. Observe how this too will respond to changes in the selected earthquake.
def bivariate(ds):
return ds.data.hvplot.bivariate('mag', 'depth')
filtered_ds.apply(bivariate)
Connecting widgets to the pipeline#
At this point you may be thinking that we haven’t done anything we haven’t already seen in the previous sections. However, apart from automatically handling the caching of computations, building visualization pipelines in this way provides one major benefit - we can inject parameters at any stage of the pipeline. These parameters can come from anywhere including from Panel widgets, allowing us to expose control over any aspect of our pipeline.
You may have noticed that the earthquakes_around_point function takes two arguments, the index of the point and the degrees_dist, which defines the size of the region around the selected earthquake we will select points in. Using .apply we can declare a FloatSlider widget and then inject its value parameter into the pipeline (ensure that an earthquake is selected in the map above):
dist_slider = pn.widgets.FloatSlider(name='Degree Distance', value=0.5, start=0.1, end=2)
filtered_ds = dataset.apply(earthquakes_around_point, index=index_stream.param.index,
degrees_dist=dist_slider)
pn.Column(
dist_slider,
pn.Row(
filtered_ds.apply(histogram),
filtered_ds.apply(scatter)))
When the widget value changes the pipeline will re-execute the part of the pipeline downstream from the function and update the plot. This ensures that only the parts of the pipeline that are actually needed are re-executed.
The .apply method can also be used to apply options depending on some widget value, e.g. we can create a colormap selector and then use .apply.opts to connect it to the rasterized_pop plot:
cmaps = {n: cc.palette[n] for n in ['kbc', 'fire', 'bgy', 'bgyw', 'bmy', 'gray', 'kbc']}
cmap_selector = pn.widgets.Select(name='Colormap', options=cmaps)
rasterized_pop_cmapped = rasterized_pop.apply.opts(cmap=cmap_selector)
pn.Column(cmap_selector, rasterized_pop_cmapped)
Exercise#
Use the .apply.opts method to control the style of some existing component, e.g. the size of the points in the dynamic_scatter plot or the color of the dynamic_histogram.
(Hint)
Use a ColorPicker widget to control the color or a FloatSlider widget to control the size.
color_picker = pn.widgets.ColorPicker(name='Color', value='#00f300')
size_slider = pn.widgets.FloatSlider(name='Size', value=5, start=1, end=30)
color_histogram = dynamic_histogram.apply.opts(color=color_picker.param.value)
size_scatter = dynamic_scatter.apply.opts(size=size_slider.param.value)
pn.Column(
pn.Row(color_picker, size_slider),
pn.Row(color_histogram, size_scatter)
)
Connecting panels to streams#
At this point we have learned how to connect parameters on Panel objects to a pipeline and we earlier learned how we can use parameters to declare dynamic Panel components. So, this section should be nothing new; we will simply try to connect the index parameter of the selection stream to a panel to try to compute the number of people in the region around an earthquake.
Since we have a population density dataset we can approximate how many people are affected by a particular earthquake. Of course, this value is only a rough approximation, as it ignores the curvature of the earth, assumes isotropic spreading of the earthquake, and assumes that the population did not change between the measurement and the earthquake.
def affected_population(index, distance):
if not index:
return "No earthquake was selected."
sel = most_severe.iloc[index[0]]
lon, lat = sel.longitude, sel.latitude
lon_dist = (np.cos(np.deg2rad(lat)) * 111.321543) * distance
lat_dist = 111.321543 * distance
hdist = distance / 2.
mean_density = cleaned_ds.sel(x=slice(lon-hdist, lon+hdist), y=slice(lat+hdist, lat-hdist)).mean().item()
population = (lat_dist * lon_dist) * mean_density
return 'Approximate population around {place}, where a magnitude {mag} earthquake hit on {date} is {pop:.0f}.'.format(
pop=population, mag=sel.mag, place=sel.place, date=sel.name)
def bounds(index, value):
if not index:
return hv.Bounds((0, 0, 0, 0))
sel = most_severe.iloc[index[0]]
hdist = value / 2.
lon, lat = sel.longitude, sel.latitude
return hv.Bounds((lon-hdist, lat-hdist, lon+hdist, lat+hdist))
dynamic_bounds = hv.DynamicMap(bounds, streams=[index_stream, dist_slider.param.value])
bound_affected_population = pn.bind(affected_population, index=index_stream.param.index, distance=dist_slider)
pn.Column(pn.panel(bound_affected_population, width=400),
rasterized_pop * high_mag_points * dynamic_bounds, dist_slider)
The full dashboard#
Finally let us put all these components together into an overall dashboard, which we will mark as servable so we can panel serve this notebook.
title = '## Major Earthquakes 2000-2018'
logo = pn.panel(pathlib.Path('../assets/usgs_logo.png'), width=200, align='center')
widgets = pn.WidgetBox(dist_slider, cmap_selector, margin=5)
header = pn.Row(pn.Column(title, pn.panel(bound_affected_population, width=400)),
pn.layout.Spacer(width=10), logo, pn.layout.HSpacer(), widgets)
dynamic_scatter = filtered_ds.apply(scatter)
dynamic_histogram = filtered_ds.apply(histogram)
temporal_vline = hv.DynamicMap(vline_callback, streams=[index_stream])
rasterized_pop_cmapped = rasterized_pop.apply.opts(cmap=cmap_selector.param.value)
dynamic_bounds = hv.DynamicMap(bounds, streams=[index_stream, dist_slider.param.value])
body = pn.Row(
rasterized_pop_cmapped * high_mag_points * dynamic_bounds,
pn.Column(dynamic_scatter * temporal_vline, dynamic_histogram),
)
pn.Column(header, body).servable()
Conclusion#
If you have gone through all the tutorials and exercises, you should now have a very good idea of the power of the HoloViz ecosystem, and how each of the tools fit together. You can see many examples of HoloViz apps at examples.pyviz.org, though do note that each of them was written at a particular stage of HoloViz development and may not be using the best-practice recommendations as outlined in these tutorials. Have fun working with the tools, and feel free to chime in at our Discourse site if you have usage questions that others in the community can answer!
