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@section('title')
    Bar Chart Examples from Lab 9
@stop

@section('content')

# Lab 9: Bar charts

#### Reference
+ [matplotlib.pyplot.bar](http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.bar)
+ [matplotlib.pyplot.barh](http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.barh)

#### Preface
For all of the examples below, assume both numpy and pyplot have been imported:
```py
import numpy as np
import matplotlib.pyplot as plt
```

#### Sample data
To demonstrate how to make bar charts, we're going to chart some sample data mapping students favorite candy.

```py
counts = [5,20,12]
candy = ["skittles","twizzlers","m&ms"]
```


## Super simple bar chart Version 1.0

```py
counts = [5,20,12]
candy  = ["skittles","twizzlers","m&ms"]

position = np.arange(len(counts)) # [0 1 2]

# Default color is blue, default width is 0.8

#          x        y
plt.bar(position,counts)

show()
```

Note there are no labels in this example, just the bars of height `5`, `20`, and `12`.

We essentially plotted `[0,1,2]` against `[5,20,12]`.

<img src="candy1.png" style="max-width:400px">


## Super simple bar chart Version 2.0

```py
counts = [5,20,12]
candy  = ["skittles","twizzlers","m&ms"]

position = np.arange(len(counts)) # [0 1 2]

# New width, color and alignment of bars
plt.bar(position, counts, width=0.5, color='green', align='center')

plt.show()
```

<img src="bar2.png" style="max-width:400px">

In this example we changed the color, width and placement of the bars (all in the `bar()` function)



## Super simple bar chart Version 3.0

```py
counts = [5,20,12]
candy  = ["skittles","twizzlers","m&ms"]

position = np.arange(len(counts)) # array([0,1,2])

plt.bar(position,counts,width=0.5,color='green',align='center')

plt.xticks(position,candy) # added labels to x-axis

plt.title('Candy votes') # added title

plt.show()
```

<img src="bar3.png" style="max-width:400px" >

In this example we added labels by using `plt.xticks` and a title using `plt.title`.


## A more complex bar chart

```py
# Made up data:
data  = [79,83,92,43,64,78,85,93]
names = ["AB","BG","KG","CW","WE","GD","TM","OL"]

grid(True) # background grid visible

# `arange()` is like range but returns an array
# and works for floating point numbers
position = np.arange(len(data))

plt.bar(position, data, width=0.5, color='darkolivegreen', align='center')

xticks(position,names) # Marks on x-axis

title('Sample bar chart') # Title of bar chart

plt.show()
```

<img src="pp2.png" style="max-width:600px">



## A horizontal bar chart
All of the above examples used `bar()` which creates a vertical bar chart. To make a horizontal bar chart, use [`barh()`](http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.barh).

Example:

```py
counts = [5,20,12]
candy  = ["skittles","twizzlers","m&ms"]

position = np.arange(len(candy))

plt.barh(position,counts,0.5,align='center')

plt.yticks(position,candy) # Marks on x-axis

plt.show()
```

<img src="barh-example.png" style="max-width:587px">




@include('/labs/lab09/_toc')

@stop