# Getting Started with ggplot2 in R

## Grammar

A grammar provides a foundation for understanding different types of

graphics. A grammar may also help us on what a well-formed or

correct graphic looks like, but there will still be many grammatically

correct but nonsensical graphics. This is easy to see by analogy to the

English language: good grammar is just the first step in creating a good

sentence.

## Grammar of Graphics

A grammar of graphics is a tool that enables us to clearly

describe the components of a graphics. Such a grammar allows us to move

beyond named graphics (e.g., the “scatterplot”) and gain insights into

the deep structures that underlies the statistical graphics. `ggplot2`

.

proposes an alternative parameterization of the grammar, based around

the idea of building up a graphic from multiple layers of data.

## Components of ggplot2

- Data and aesthetic mappings
- Geometric objects
- Scale
- Facet Specification
- Statistical Transformation
- Coordinate Syatem

## Layered grammar of graphics

Together, the data, mappings, statistical transformations and geometric

objects form a layer. Plot may have different layers. Layers are

responsible for creating the objects that we expect on the plots.

## How to use ggplot2 in R?

For this we need to have installed ggplot2 package in our IDE. Let us

use ggplot in R builtin data `diamonds`

.

```
library(ggplot2)
ggplot(diamonds, aes(carat,price)) + geom_point()
```

`geom_point`

is used for scatter plot. From above figure, we can see that whenever

diamond’s carat increases, prices also increases. We can not see

how the data distibuted for this, let's make some changes in our code.

```
ggplot(diamonds,aes(carat,price)) + geom_point() +
scale_x_continuous() + scale_y_continuous()
```

We can see better distribution of points than previous plot. We can clearly see

that carat and price variable are not linearly distributed. To make it

linearly distributable, lets make some changes in our code.

```
ggplot(diamonds, aes(carat,price)) + geom_point() +
stat_smooth(method = lm) + scale_x_log10() + scale_y_log10()
```

`## ``geom_smooth()`

using formula 'y ~ x'

From above graph, relationship between price and carat variables is

linear. If we try the code without `stat_smooth(method= lm)`

we can not

see linear line in graph. Where lm means linear model.

```
ggplot(diamonds, aes(carat,price)) + geom_point() +
scale_x_log10() + scale_y_log10()
```

## Lets make histogram of `diamonds`

data

`ggplot(diamonds, aes(price)) + geom_histogram()`

`## ``stat_bin()`

using `bins = 30`

. Pick better value with `binwidth`

.

To build histogram, we use function `geom_histogram()`

. We should note that

histogram is made on one dimensional data. If we want to add title of

the plot we can do as,

`ggplot(diamonds, aes(price)) + geom_histogram() + ggtitle("ggplot2 Histogram")`

`## ``stat_bin()`

using `bins = 30`

. Pick better value with `binwidth`

.

## Let us try some other ggplot2 features in R builtin data `mtcars`

```
ggplot(mpg, aes(x = displ, y = hwy)) +
geom_point()
```

The figure above shows scatterplot between`hwy`

and`displ`

variables of `mtcars`

data from figure we can see as the values of`hwy`

increases, values of`displ`

variable slightly decreases.

## Let’s add `geam_smooth()`

: What will happen?

`ggplot(mpg, aes(displ, hwy)) + geom_point() + geom_smooth()`

`## ``geom_smooth()`

using method = 'loess' and formula 'y ~ x'

We see there is smooth line appearing on the middle of the data points.

## Adding “wiggliness” in the smoothing plot

`ggplot(mpg, aes(displ, hwy)) + geom_point() + geom_smooth(span = 0.2) `

`## ``geom_smooth()`

using method = 'loess' and formula 'y ~ x'

What changes can we see in above graph and previous graph. Let us again

check by keeping `span = 1`

.

`ggplot(mpg, aes(displ, hwy)) + geom_point()+ geom_smooth(span = 1)`

`## ``geom_smooth()`

using method = 'loess' and formula 'y ~ x'

We can make sense that, by default ggplot kept value of span 1. If we set` method= lm`

we can find stright smooth line. Let us` inside `

geom_smooth()

try.

`ggplot(mpg, aes(displ, hwy)) + geom_point() + geom_smooth(method = lm) `

`## ``geom_smooth()`

using formula 'y ~ x'

## Let Modify our code little,

`ggplot(mpg, aes(displ, hwy)) + geom_point() + geom_smooth(method = lm, se= FALSE) `

`## ``geom_smooth()`

using formula 'y ~ x'

By `se= FALSE`

we added a smooth line.

## Fixed color

`ggplot(mpg, aes(displ,hwy)) + geom_point(color = 'red')`

## Changing color by variable attributes

Lets change our color based on class.

```
ggplot(mpg, aes(displ, hwy, colour = class)) +
geom_point()
```

Here we gave colors according to variable's name.

## Getting multiple scatterplot of attributes

We can get multiple scatterplot by using `facet_wrap()`

function.

```
ggplot(mpg, aes(displ, hwy)) + geom_point() +
facet_wrap(~class)
```

In above figure, we found distribution of various variables along with `displ`

and `hwy`

variables.

## Histogram

`ggplot(mpg, aes(hwy)) + geom_histogram()`

`## ``stat_bin()`

using `bins = 30`

. Pick better value with `binwidth`

.

`hwy`

variable bins automatically.

## Changing bin size of the histogram

`ggplot(mpg, aes(hwy)) + geom_histogram(binwidth = 2.5)`

## Frequency polygon

A frequency polygon is a line graph of class

frequency plotted against class midpoint. It can be obtained by joining

the midpoints of the top of the rectangles in the histogram.

`ggplot(mpg, aes(hwy)) + geom_freqpoly()`

`## ``stat_bin()`

using `bins = 30`

. Pick better value with `binwidth`

.

## Change Bin size of frequency Polygon

`ggplot(mpg, aes(hwy)) + geom_freqpoly(binwidth= 1)`

We can see the effect of binwidth from figure by comparing above figure

with previous one.

## Histogram with faceting:

We have already discussed about what a facet does in scatter plot. Similarly, in

histogram it gives multiple subplots.

```
ggplot(mpg, aes(displ, fill = drv)) + geom_histogram(binwidth = 0.5) +
facet_wrap(~drv, ncol = 1)
```

## Bar plot

`ggplot(mpg, aes(manufacturer)) + geom_bar()`

We can draw bar plot in `geom_bar()`

function. From bar plot we can see

`dodge`

and `toyotao`

has maximum frequency.

## Let’s Use alpha inside `geom_point()`

`ggplot(mpg, aes(cty, hwy)) + geom_point(alpha = 1 / 3)`

Alpha refers to the opacity of a geom. Values of alpha range from 0 to

1, with lower values corresponding to more transparent colors.

## Modifying the axes

```
ggplot(mpg, aes(cty, hwy)) +geom_point(alpha = 1 / 3) + xlab("city driving (mpg)") +
ylab("highway driving (mpg)")
```

```
ggplot(mpg, aes(cty, hwy)) + geom_point(alpha = 1 / 3) + xlab(NULL) +
ylab(NULL)
```

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