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Data Science using Python Tutorials

Introduction to data visualization using matplot

Matplotlib is one of the most popular Python packages used for data visualization. It is a cross-platform library for making 2D plots from data in arrays. It provides an object-oriented API that helps in embedding plots in applications using Python GUI toolkits such as PyQt,  WxPython Tkinter. It can be used in Python and IPython shells,  Jupyter notebook, and web application servers also. 

How to install matplot 

Matplotlib and its dependency packages are available in the form of wheel packages on the standard Python package repositories and can be installed on Windows, Linux as well as macOS systems using the pip package manager. 

python -m pip install -U pip  
python -m pip install -U matplotlib 

matplotlib.pyplot is a collection of command style functions that make  Matplotlib work like MATLAB. Each pyplot function makes some changes to a figure. For example, a function creates a figure, a plotting area in a figure, plots some lines in a plotting area, decorates the plot with labels, etc. 

The following are the list of plots that are available in matplot  

Plot -It is used to  plot lines and/or markers to the Axes 

Scatter Make a scatter plot of x vs y 

Bar Make a bar plot 

Barh Make a horizontal bar plot 

Boxplot Make a box and whisker plot 

Hist Plot a histogram 

Pie Plot a pie chart 

Step Make a step plot 

Stem Create a stem plot 

Stackplot Draw a stacked area plot

The following are the list of Axis Functions that are available in  matplot  

Axes Add axes to the figure 

Text Add text to the axes. 

Title Set a title of the current axes 

X – label Set the x-axis label of the current axis 

Y – label Set the y-axis label of the current axis 

X – lim Get or set the x limits of the current axes 

Y – lim Get or set the y limits of the current axes 

X – ticks Get or set the x-limits of the current tick locations and labels. Y – ticks Get or set the y-limits of the current tick locations and labels.

The following are the list of Figure Functions that are available in  matplot  

Figtext Add text to figure 

Figure Creates a new figure

Figtext Add text to figure

Sample Plot using matplot  

Create a new untitled notebook with the .ipynb extension (stands for  the IPython notebook) 

We shall now display a simple line plot between 2 arrays To begin with, the Pyplot module from the Matplotlib package is imported,  with an alias plt as a matter of convention.

import matplotlib.pyplot as plt  

Next we need an array of numbers to plot. Various array functions are  defined in the NumPy library which is imported with the np alias.

import numpy as np  

Now we will obtain the ndarray objects for x and y 

x = np.random.randint(5, 50,10)

## To generate random numbers for plotting

y = 2*x

The values from two arrays are plotted using the plot()

function. plt.plot(x,y)  

You can set the plot title, and labels for x and y axes.

plt.xlabel("X-values")
plt.ylabel("Y-values")
plt.title('X vs Y’)  

The Plot viewer window is invoked by the show() function plt.show() 

The complete program is as follows  

import numpy as np  
import matplotlib.pyplot as plt  
x=np.random.randint(5,50,10)  
y=2*x  
plt.xlabel("X-values")  
plt.ylabel("Y-values")  
plt.title('X vs Y')  
plt.plot(x,y) 

Output:  

Sample Plot using matplot

To display plot outputs inside the notebook itself and not in the  separate viewer , enter the following magic statement

%matplotlib inline 

Matplotlib – PyLab module  

PyLab is a procedural interface to the Matplotlib object-oriented plotting library. Matplotlib is the whole package; matplotlib.pyplot is a  module in Matplotlib, and PyLab is a module that gets installed alongside Matplotlib. 

PyLab is a convenience module that imports matplotlib.pyplot (for plotting) and NumPy (for Mathematics and working with arrays) in a  single namespace. Although many examples use PyLab, it is no longer recommended. Now we will plot a curve and try to play with the color and markers style

import numpy as np  
import matplotlib.pyplot as plt  
x = np.linspace(-3, 3, 30)  
y = x**2  
plt.plot(x,y)

Output: 

Matplotlib - PyLab module

Supported marker styles :

 “+” , “,” , “.” , “1” , “2” , “3” , “4”  Supported line styles : 

” , “– –” , “-.” , “:” , “steps”  

Supported colour formats : 

b” , “g” , “r” , “c” , “m” , “y” , “k” , “w”  

To plot symbols rather than lines and change colour , we need to  provide an additional string argument. 

import numpy as np  
import matplotlib.pyplot as plt  
x = np.linspace(-3, 3, 30)  
y = x**2  
## “g” represents the colour of the graph  
plt.plot(x,y,’g’)  

Output:

Supported colour formats

Now we will change the marker style of the above graph.

import numpy as np  
import matplotlib.pyplot as plt  
x = np.linspace(-3, 3, 30)  
y = x**2  
## “r” represents the colour and “-o” represents the marker style plt.plot(x,y,’r-o’)

Output:  

data visualization using matplot

Plotting curves of given equation  

Here, we use NumPy which is a general-purpose array-processing  package in python. 

• To set the x-axis values, we use the np.arange() method in which the first two arguments are for range and the third one for step-wise increment. The result is a numpy array. 

• To get corresponding y-axis values, we simply use the predefined np.sin() method on the numpy array. 

• Finally, we plot the points by passing x and y arrays to the  plt.plot() function.

x = np.arange(0, 2*(np.pi), 0.1)  y = np.sin(x)  
plt.plot(x, y)

Output:  

Plotting curves of given equation

x = np.linspace(-3, 3, 30)  
y = np.linspace(-3, 3, 20)  
plt.plot(x, sin(x),'g--')  plt.scatter(y,sin(y),c='b',marker='o')  plt.show()

Output: 

data visualization using matplot

Plots can be overlaid. Just use the multiple plot commands.

x = np.linspace(-3, 3, 30)  
plt.plot(x, sin(x),’b’)  
plt.plot(x, cos(x), ‘r’)  
plt.plot(x, -sin(x), ‘g’)  
plt.legend([‘Sin’,’Cos','-Sin'])

Output:  

data visualization using matplot

• Here, we plot three curves on the same graph. We differentiate between them by giving them a name(label) which is passed as an argument of  .plot() function. 

• The small rectangular box giving information about the type of line and its color is called a legend. We can add a legend to our plot  using .legend() function.

Multiple Subplots  

Sometimes it is helpful to compare different views of data side by side.  To this end, Matplotlib has the concept of subplots: groups of smaller axes that can exist together within a single figure. These subplots might be insets, grids of plots, or other more complicated layouts. In this section, we’ll explore four routines for creating subplots in Matplotlib. This is the general syntax for subplots  

subplot(nrows, ncols, index)  

Now we will try to create a empty 2 X 2 layout

plt.subplots(2,2)

Output:

Multiple Subplots

Now we will try to plot graphs using subplot function 

x = np.linspace(-3, 3, 30)  
plt.subplot(2,2,1)  
plt.plot(x, sin(x),’b’)  
plt.subplot(2,2,2)  
plt.plot(x, cos(x),’r’)  
plt.subplot(2,2,3)  
plt.plot(x, -sin(x),’g’)  
plt.subplot(2,2,4)  
plt.plot(x, -cos(x),’y')  

Output: 

data visualization using matplot

In the next article, we will learn how to visualize the remaining types of plots.

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