Simple Visualizations with data.vgplot

The core interface of pdvega is the vgplot attribute that it adds to Pandas DataFrame and Series objects:

import pdvega

Like the plot attribute that is built-in to Pandas, there are two ways of creating plots with vgplot: first, you can call the vgplot attribute of a Pandas object directly:

from vega_datasets import data
iris = data.iris()

iris.vgplot(kind='scatter', x='sepalLength', y='petalLength', c='species')

Equivalently, you can call the specific method associated with each plot type:

iris.vgplot.scatter(x='sepalLength', y='petalLength', c='species')

The benefit of the second approach is that it allows exploration of available plot types via tab completion, and the individual functions also provide more detailed documentation of the arguments available for each method.

The vgplot interface exposes nine basic plot types; we will show examples of these below.

Datasets

For the examples on this page, we will use a number of datasets made available in the vega_datasets package:

iris = data.iris()
stocks = data.stocks(pivoted=True)
cars = data.cars()

These datasets are stored in the form of pandas dataframes:

>>> iris.head()
   petalLength  petalWidth  sepalLength  sepalWidth species
0          1.4         0.2          5.1         3.5  setosa
1          1.4         0.2          4.9         3.0  setosa
2          1.3         0.2          4.7         3.2  setosa
3          1.5         0.2          4.6         3.1  setosa
4          1.4         0.2          5.0         3.6  setosa

>>> stocks.head()
symbol       AAPL   AMZN  GOOG     IBM   MSFT
date
2000-01-01  25.94  64.56   NaN  100.52  39.81
2000-02-01  28.66  68.87   NaN   92.11  36.35
2000-03-01  33.95  67.00   NaN  106.11  43.22
2000-04-01  31.01  55.19   NaN   99.95  28.37
2000-05-01  21.00  48.31   NaN   96.31  25.45


>>> cars.head()
   Acceleration  Cylinders  Displacement  Horsepower  Miles_per_Gallon  \
0          12.0          8         307.0       130.0              18.0
1          11.5          8         350.0       165.0              15.0
2          11.0          8         318.0       150.0              18.0
3          12.0          8         304.0       150.0              16.0
4          10.5          8         302.0       140.0              17.0

                        Name Origin  Weight_in_lbs        Year
0  chevrolet chevelle malibu    USA           3504  1970-01-01
1          buick skylark 320    USA           3693  1970-01-01
2         plymouth satellite    USA           3436  1970-01-01
3              amc rebel sst    USA           3433  1970-01-01
4                ford torino    USA           3449  1970-01-01

Line Plots with vgplot.line

The default plot type for vgplot is a line plot:

stocks.vgplot()

Unless otherwise specified, the index of the DataFrame or series is used as the x-axis variable, and a separate line will be created for the y-values in each column in the dataframe. If you’d like to plot a subset of the columns, you can use pandas indexing to select the columns you are interested in:

stocks[['AAPL', 'AMZN']].vgplot.line()

Optionally, you can specify the column names to use for the x-axis and y-axis:

stocks.vgplot.line(x='AAPL', y='AMZN')

Line plots can be further customized; see the function documentation for more information:

Scatter Plots with vgplot.scatter

The previous plot might make more sense in the form of a scatter plot. This can be done with vgplot.scatter():

stocks.vgplot.scatter(x='AAPL', y='AMZN')

You can also encode the color and size of scatter plots; let’s switch to the cars dataset to see the relationship between some of these variables:

cars.vgplot.scatter(x='Horsepower', y='Miles_per_Gallon',
                    c='Origin', s='Weight_in_lbs', alpha=0.5)

This is one slight difference from the Pandas plot interface: in Pandas the c and s parameters must be passed as arrays, while here we pass them as column names.

Scatter plots can be further customized; see pdvega.FramePlotMethods.scatter() for more information.

Area Plots with vgplot.area

Area plots are quite similar to line plots, but curves are filled and stacked, meaning the top curve reflects the sum of all the ones below:

stocks[['MSFT', 'AAPL', 'AMZN']].vgplot.area()

Area charts can also be unstacked and overlaid, in which case transparency can be useful:

stocks[['MSFT', 'AAPL', 'AMZN']].vgplot.area(stacked=False, alpha=0.4)

Area plots can be further customized; see the function documentation for more information:

Bar Charts with vgplot.bar

Bar charts are supported using vgplot.bar(). Let’s create a small dataset to use for this:

import numpy as np
import pandas as pd
np.random.seed(1234)

df = pd.DataFrame(np.random.rand(10, 2), columns=['a', 'b'])

df.vgplot.bar()

Multiple bar plots will be layered on top of each other; like with area charts, they can be stacked using the stacked=True option:

df.vgplot.bar(stacked=True)

Additionally, horizontal bar plots can be created with barh:

df.vgplot.barh(stacked=True)

Bar charts can be further customized; see the function documentation for more information:

Histograms with vgplot.hist

Histograms can be created with the vgplot.hist() method.

Let’s create some data to make some distributions:

import pandas as pd
import numpy as np
df = pd.DataFrame({'a': np.random.randn(1000) + 1,
                   'b': np.random.randn(1000),
                   'c': np.random.randn(1000) - 1},
                  columns=['a', 'b', 'c'])

We’ll specify 50 bins, and create a layered histogram with a 50% transparency:

df.vgplot.hist(bins=50, alpha=0.5)

Alternatively, we can stack the histogram, and use histtype to specify that we want a filled step chart rather than a bar chart:

df.vgplot.hist(histtype='stepfilled', stacked=True, bins=50)

Histograms can be further customized; see the function documentation for more information:

KDE/Density plots with vgplot.kde

Similar to a histogram is a kernel density estimation plot (kde) which creates a smooth curve representing the density of points. This can be created with the vgplot.kde method. We’ll use the same data we did in the histogram section:

df.vgplot.kde()

KDE plots can be further customized; see the function documentation for more information:

Pie Charts

No.

Heatmaps

Pandas plotting has a function to create a hexagonally-binned heatmap of two-dimensional data. Unfortunately neither Vega nor Vega-Lite currently support hexagonal binning. But they do support cartesian heatmaps, and this functionality is included in pdvega:

df.vgplot.heatmap(x='a', y='b', gridsize=20)

Here the gridsize parameter indicates approximately how many grid points span the plot. Alternatively, instead of computing the count within each bin, we can compute the mean of a third column, specified by the C parameter:

df.vgplot.heatmap(x='a', y='b', C='c', gridsize=20)

Heatmap plots can be further customized; see pdvega.FramePlotMethods.heatmap() for more information.

Other Plot Types

The above plots are the basic plot types supported by pdvega; more sophisticated plots are available in the pdvega.plotting module. For examples of these, refer to Statistical Visualization with pdvega.plotting.