Visualizing data – overlaying charts in python

Visualizing data is vital to analyzing data.  If you can’t see your data – and see it in multiple ways – you’ll have a hard time analyzing that data.  There are quite a few ways to visualize data and, thankfully, with pandas, matplotlib and/or seaborn, you can make some pretty powerful visualizations during analysis.

One of the things I like to do when I get a new dataset is try to visualize data points against each other to see if there’s anything that jumps out at me.   To do this, I like to overlay charts against each other to find any patterns in the data / charts. With matplotlib, this is pretty easy to do but working with dual-axis can be a bit confusing at first.


Want  to learn more about data visualization and/or matplotlib? Here are a few books / websites with good info on the topic.


One chart that I like to look at for data that I know has a relationship – like sales revenue and number of widgets sold – is the dual overlay of revenue vs quantity.  An example of one of my go-to approaches for visualizing data is in Figure 1 below.

Visualizing data - revenue vs number of items
Figure 1: Visualizing data — Revenue vs Quantity chart overlay

In this chart, we have Monthly Sales Revenue (blue line) chart overlay-ed against the Number of Items Sold chart (multi-colored bar chart). This type of chart lets me quickly see if there are any easy patterns in the revenue vs # of items.

I’ve not found a quick/easy way to build the multi-colored bar chart without hacking the data and building each colored section manually…so if you know a better way that what I share below, let me know.

An example

Here’s my code for building this chart using this data.

This is just one way of visualizing data with python. Hopefully its a good example of a different approach that you may not have thought about.

Eric D. Brown , D.Sc. has a doctorate in Information Systems with a specialization in Data Sciences, Decision Support and Knowledge Management. He writes about utilizing python for data analytics at pythondata.com and the crossroads of technology and strategy at ericbrown.com

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