Blog - basic scatter plot

_posts/2018-08-15-altair-to-mpl-scatter-part1.md

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+---
+layout: post
+title:  "Making a Scatter Plot - Part 1"
+date:   2018-08-15 15:00:00 -0500
+author: "Kimberly Orr", "Nabarun Pal"
+categories: user-guide
+tags: "intro about scatter"
+excerpt_separator: <!--read more-->
+---
+
+# Making a Scatter Plot
+This post will walk you through making a scatter plot in Altair, Matplotlib, and mpl-altair.
+We'll examine the cars dataset from [Vega's collection of datasets](https://github.com/vega/vega-datasets#vega-datasets):
+```python
+from vega_datasets import data
+cars = data.cars()
+cars.head()
+```
+**Acceleration** | **Cylinders** | **Displacement** | **Horsepower** | **Miles_per_Gallon** | **Name** | **Origin** | **Weight_in_lbs** | **Year** 
+:---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---:
+12.0 | 8 | 307.0 | 130.0 | 18.0 | chevrolet chevelle malibu | USA | 3504 | 1970-01-01
+11.5 | 8 | 350.0 | 165.0 | 15.0 | buick skylark 320 | USA | 3693 | 1970-01-01
+11.0 | 8 | 318.0 | 150.0 | 18.0 | plymouth satellite | USA | 3436 | 1970-01-01
+12.0 | 8 | 304.0 | 150.0 | 16.0 | amc rebel sst | USA | 3433 | 1970-01-01
+10.5 | 8 | 302.0 | 140.0 | 17.0 | ford torino | USA | 3449 | 1970-01-01
+
+Let's plot Horsepower vs Weight from the cars dataset.
+
+## Altair
+```python
+import altair as alt
+alt.Chart(cars).mark_point().encode(
+    alt.X('Weight_in_lbs'),
+    alt.Y('Horsepower')
+)
+```
+![png](pics/altair-to-mpl-scatter-part1_0.png)
+
+## Matplotlib
+ax.scatter() allows you to specify that your data is being specified as columns from a dataframe via the `data` parameter.
+
+Notice that we have to manually specify the axis labels, the gridlines, and the fact that we want to include zero as our lower limit (Altair defaults to include zero on quantitative axes).
+```python
+import matplotlib.pyplot as plt
+fig, ax = plt.subplots()
+ax.scatter('Weight_in_lbs', 'Horsepower', data=cars)
+ax.set_xlabel('Weight_in_lbs')
+ax.set_ylabel('Horsepower')
+ax.set_xlim([0, None])
+ax.set_ylim([0, None])
+plt.grid()
+plt.show()
+```
+![png](pics/altair-to-mpl-scatter-part1_1.png)
+
+
+## mpl-altair
+To produce the mpl-altair plot, create the Altair chart and then pass it to 
+mplaltair's `convert()` function. The resulting fig and ax are Matplotlib objects.
+```python
+import altair as alt
+import matplotlib.pyplot as plt
+import mplaltair
+chart = alt.Chart(cars).mark_point().encode(
+    alt.X('Weight_in_lbs'),
+    alt.Y('Horsepower')
+)
+fig, ax = mplaltair.convert(chart)
+plt.show()
+```
+![png](pics/altair-to-mpl-scatter-part1_2.png)
+
+Why is it beneficial to convert the Altair chart to a Matplotlib figure?
+- the figure can be modified using Matplotlib functionality, which has more functionality than Altair (by design)
+- the figure can be rendered without Altair's front-end [renderers](https://altair-viz.github.io/user_guide/renderers.html).