Bar Graph using HTML Canvas



We will be using rectangles to draw Bar on our Canvas.

details of rectangles by changing its Height width and coordinates.

Data Format

We have class name, total Student in class as our graph. In Class-4 , we have total 150 students. We will create one JavaScript array with these data for all the classes.
var data = [['Class-5',140],['Class-4',150],['Class-3',170],['Class-2',110],['Class-1',170]];
You can extend this data by adding more classes but you need to adjust width of the Canvas to accommodate all. We can adjust the width automatically by reading number of data we have , width of each graph and gap between the bars .
my_canvas.width=data.length * (  bar_gap)  + x  ;
We kept our data in an array, we will use one for loop to draw bars representing student strength in a class.
Inside the loop after drawing one bar (representing strength of a class ) we will move distance equal to bar_gap in X axis. In each step we calculate different values for x1 like this.
x1=x;
....
x=x+bar_gap;

Understanding fillRect() for drawing Bars

We can draw rectangles as bars representing student data of the class. Note that rectangle coordinates starts from top left corners of the rectangle. Bar Graph settings on HTML Canvas
y1=y-data[i][1]; // Coordinate for top of the Bar 
x1=x; 
gctx.fillRect(x1, y1, width, height );
Here x1 , y1 are coordinates of the top left of the rectangle.

Text ( Class name ) on Graph

We will write name of the class below the bar graph representing the class strength. Here base line is at a distance of y from top edge of the graph. We will move little down ( y – axis ) from this base line and display the text. Our class is available inside array as data[i][0].
gctx.font = '18px serif'; // font for base label showing classes 
gctx.textAlign='left';
gctx.textBaseline='top';
gctx.fillStyle= '#008cf5';
gctx.fillText(data[i][0], x,y+5); // Write base text for classes

Writing value at top of graph

The top left corner of the bar graph is represented by x1, y1. We will move little up by subtracting 20 from y1 ( y1-20 ) to display the value. The value is available inside array as data[i][[1]
y1=y-data[i][1]; // Coordinate for top of the Bar 
x1 = x;    
gctx.font = '12px serif'; // font at top of the bar 
gctx.fillStyle= '#000000';
gctx.fillText(data[i][1], x1,y1-20); // text at top of the bar 

Adding shadow to graph

gctx.shadowColor = '#000000';
gctx.shadowOffsetX=3;
gctx.shadowOffsetY=3;
gctx.shadowBlur = 3;

Full Code is here

HTML Canvas
<canvas id="my_canvas" width="35" height="230"  style="border:2px solid #000000;"></canvas>
JavaScript
<script>
var my_canvas=document.getElementById("my_canvas");
var gctx=my_canvas.getContext("2d");

var data = [['Class-5',140],['Class-4',150],['Class-3',170],['Class-2',110],['Class-1',170]];

///////// Settings  ////////// 

var bar_width=50;
var y_gap=30;  // Gap below the graph 
var bar_gap=100; // Gap between Bars including width of the bar
var x= 20; // Margin of graph from left  

y=my_canvas.height -y_gap ;

my_canvas.width=data.length * (  bar_gap)  + x  ;
////////////end of settings ////
gctx.moveTo(x-5,y);
gctx.lineTo(my_canvas.width,y); // Base line of graph 
gctx.stroke();
/// add shadow ///
gctx.shadowColor = '#000000';
gctx.shadowOffsetX=3;
gctx.shadowOffsetY=3;
gctx.shadowBlur = 3;
/////////// Draw the graph ////////

for (i=0;i<data.length;i++){
gctx.shadowColor = '#ffffff'; // remove this line if you shadow on text is required
gctx.font = '18px serif'; // font for base label showing classes 
gctx.textAlign='left';
gctx.textBaseline='top';
gctx.fillStyle= '#008cf5';
gctx.fillText(data[i][0], x,y+5); // Write base text for classes 

gctx.beginPath();
gctx.lineWidth=2;
y1=y-data[i][1]; // Coordinate for top of the Bar 
x1 = x;    
gctx.font = '12px serif'; // font at top of the bar 
gctx.fillStyle= '#000000';
gctx.fillText(data[i][1], x1,y1-20); // text at top of the bar 

gctx.fillStyle= '#f52369'; // fill Colur of bar  
gctx.shadowColor = '#000000'; // shadow color for bars 
gctx.fillRect(x1,y1,bar_width,data[i][1]);// Filled bar 

x=x+bar_gap

}// end of for loop 
</script>




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