# Working with large data using datashader ¶

The various plotting backends supported by HoloViews (such as Matplotlib and Bokeh) each have limitations on the amount of data that is practical to work with, for a variety of reasons. For instance, Bokeh mirrors your data directly into an HTML page viewable in your browser, which can cause problems when data sizes approach the limited memory available for each web page in your browser.

Luckily, a visualization of even the largest dataset will be constrained by the resolution of your display device, and so one approach to handling such data is to pre-render or rasterize the data into a fixed-size array or image before sending it to the backend. The Datashader package provides a high-performance big-data rasterization pipeline that works seamlessly with HoloViews to support datasets that are orders of magnitude larger than those supported natively by the plotting backends.

In [1]:
import numpy as np
import holoviews as hv
from holoviews.operation import decimate
hv.extension('bokeh')
decimate.max_samples=1000

def random_walk(n, f=5000):
"""Random walk in a 2D space, smoothed with a filter of length f"""
xs = np.convolve(np.random.normal(0, 0.1, size=n), np.ones(f)/f).cumsum()
ys = np.convolve(np.random.normal(0, 0.1, size=n), np.ones(f)/f).cumsum()
xs += 0.1*np.sin(0.1*np.array(range(n-1+f))) # add wobble on x axis
xs += np.random.normal(0, 0.005, size=n-1+f) # add measurement noise
ys += np.random.normal(0, 0.005, size=n-1+f)
return np.column_stack([xs, ys])

def random_cov():
"""Random covariance for use in generating 2D Gaussian distributions"""
A = np.random.randn(2,2)
return np.dot(A, A.T)

def time_series(T = 1, N = 100, mu = 0.1, sigma = 0.1, S0 = 20):
"""Parameterized noisy time series"""
dt = float(T)/N
t = np.linspace(0, T, N)
W = np.random.standard_normal(size = N)
W = np.cumsum(W)*np.sqrt(dt) # standard brownian motion
X = (mu-0.5*sigma**2)*t + sigma*W
S = S0*np.exp(X) # geometric brownian motion
return S