# Visualisation and plotting with Matplotlib¶

Matplotlib is the key Python package for producing so called publication-ready plot. It provides the basis for \(\omega radlib\)’s entire visualisation module, and is typically used together with NumPy - which is the other major \(\omega radlib\) dependency.

## Different ways to import matplotlib¶

### In a Python script¶

```
import matplotlib.pyplot as pl
```

### In an IPython notebook¶

```
# This magic just sets up matplotlib's interactive mode
%matplotlib
# So you have to explicitely import the module into the namespace
import matplotlib.pyplot as pl
```

If you want to enable `inline plotting`

(**mandatory if you use the** Virtual Machine for Cross-Platform Weather Radar Science)

```
# This magic just sets up matplotlib's interactive mode
%matplotlib inline
# So you have to explicitely import the module into the namespace
import matplotlib.pyplot as pl
```

`%matplotlib inline`

turns on “inline plotting”, where plot graphics will appear in your notebook. This has important implications for interactivity: for inline plotting, commands in cells below the cell that outputs a plot will not affect the plot. For example, changing the color map is not possible from cells below the cell that creates a plot. However, for other backends, such as qt4, that open a separate window, cells below those that create the plot will change the plot - it is a live
object in memory. If you are not using matplotlib in interactive mode at all, figures will only appear if you invoke `plt.show()`

.

If you do not want to use inline plotting, just use `%matplotlib`

instead of `%matplotlib inline`

. The Kernel has to be restarted for this change to become effective.

If you want to magically import numpy and matplotlib

```
%pylab
# or
%pylab inline
```

In the following, we use a sightly different syntax for `matplotlib inline`

. This is because the notebook needs to be convertable to a Python script where IPython magic does not work. Please don’t let this confuse you…

```
[1]:
```

```
# Instead of matplotlib inline
import matplotlib.pyplot as pl
try:
get_ipython().magic("matplotlib inline")
except:
pl.ion()
import numpy as np
```

## Simple plots and decorations¶

After these imports, inline plots should work rightaway, e.g. a simple line plot of the sinus function:

```
[2]:
```

```
x = np.arange(0,4*np.pi,0.1)
y = np.sin(x)
pl.plot(x, y)
```

```
[2]:
```

```
[<matplotlib.lines.Line2D at 0x7fd734020100>]
```

## More complex plots and fine control¶

A matplotlib plot can be understood from an object-oriented perspective. Each plot consists of a figure object (like a canvas), an axes object (like a subplot or panel), and other objects such as a title, an axis, or a colorbar.

Accordingly, a plot can be developed by creating or modifying the different objects. For example, the size of the entire plot is controlled by the `Figure`

object. Each `subplot`

in that figure corresponds to an `axes`

object.

```
[3]:
```

```
# Create the figure object
fig = pl.figure(figsize=(12,8))
# Add first axes object (of a multi-panel plot with two rows and one column)
ax = fig.add_subplot(211)
pl.plot(x, np.sin(x))
pl.title("The Sinus Function")
pl.xlabel("This is my x-axis label")
# Add second axes object
ax = fig.add_subplot(212)
pl.plot(x, np.cos(x))
pl.title("The Cosinus Function")
pl.xlabel("This is my x-axis label")
# Make sure the elements of the plot are arranged properly
pl.tight_layout()
```