Building your own Interactive Line Graph in ReactJS

Basic SVG Component

First, let's create a simple SVG component that accepts width and height as props. This will be the starting point for our graph.

import React from "react";

const LineGraph = ({ height, width }) => {
  return <svg height={height} width={width}></svg>;
};

export default LineGraph;

Adding the X-Axis

Now, let's add the X-axis, which runs horizontally across the graph. We’ll use the <line> element for this.

const drawXAxis = () => {
  const middleY = height / 2;

  return (
    <line x1={0} y1={middleY} x2={width} y2={middleY} stroke={lineColor} />
  );
};

Adding the Y-Axis

We’ll use another <line> element to draw the Y-axis, which will run vertically through the center of the graph.

const drawYAxis = () => {
  const middleX = width / 2;

  return (
    <line x1={middleX} y1={0} x2={middleX} y2={height} stroke={lineColor} />
  );
};

Plotting coordinates as a line path

The key part of a line graph is the line connecting different points. Let's plot some sample coordinates and connect them using an SVG .

const drawPath = () => {
    const pathData = coordinates
      .map((coordinate, index) =>
        index === 0
          ? `M ${coordinate.x} ${coordinate.y}`
          : `L ${coordinate.x} ${coordinate.y}`
      )
      .join(" ");

    return <path d={pathData} stroke={pathColor} fill="none" />;
  };

Option to fill area beneath the line

We can fill the area beneath the line with a color to enhance the graph. This can be done using an additional element. Consider prop isFillArea to show/hide this area.

const drawPath = () => {
  const pathData = coordinates
    .map((coordinate, index) =>
      index === 0
        ? `M ${coordinate.x} ${coordinate.y}`
        : `L ${coordinate.x} ${coordinate.y}`
    )
    .join(" ");

  const middleY = height / 2;
  const svgPath = showFillArea
    ? `${pathData} L ${width} ${middleY} L 0 ${middleY} Z`
    : pathData;
  const fillColor = showFillArea ? areaColor : "none";
  return (
    <path d={svgPath} fill={fillColor} stroke={pathColor} opacity="0.5" />
  );
};

Tracking the cursor

Let’s add a circle that follows the cursor's movement across the graph path.

We will need a reference of our SVG component to access the bounding box of the SVG element. Also a reference for our tracking-circle that will be used for tracking the cursor over the graph.

const svgRef = useRef();
const circleRef = useRef();
// ...
const drawTrackingCircle = () => {
  return (
    <circle
      ref={circleRef}
      r={6}
      fill="red"
      style={{ display: "none" }} // Initially hidden
    />
  );
};
// ...
<svg ref={svgRef} width={width} height={height}>
// ...
</svg>

Then, we need to add an event listener to our SVG element. This will listen to all our cursor movements over the graph.

useEffect(() => {
  const svgElement = svgRef.current;
  svgElement.addEventListener("mousemove", handleMouseMove);

  // clean up
  return () => svgElement.removeEventListener("mousemove", handleMouseMove);
}, []);

Next, we need a method to find the intersection coordinate between the cursor position and the path.

const getIntersectionPoint = (cursorX) => {
  // Find the segment (p1, p2) where cursorX lies between two consecutive coordinates.
  const segment = coordinates.find((p1, i) => {
    // Get the next point
    const p2 = coordinates[i + 1]; 
    // Check if cursorX falls between the two coordinates horizontally.
    return (
      p2 &&
      ((p1.x <= cursorX && p2.x >= cursorX) ||
        (p1.x >= cursorX && p2.x <= cursorX))
    );
  });

  // Return null if no valid segment is found.
  if (!segment) return null; 

  // Destructure the two coordinates in the segment.
  const [p1, p2] = [segment, coordinates[coordinates.indexOf(segment) + 1]];

  // Calculate 't' to determine the relative position between p1 and p2.
  const t = (cursorX - p1.x) / (p2.x - p1.x);

  // Interpolate the Y-coordinate using 't'.
  const y = p1.y + t * (p2.y - p1.y);

  return { x: cursorX, y };
};

Cursor movement tracker method. It uses the getIntersectionPoint method to find the current intersection coordinate.

const handleMouseMove = (event) => {
  // Get SVG position
  const svgRect = svgRef.current.getBoundingClientRect();
  // Calculate cursor's X within the SVG
  const cursorX = event.clientX - svgRect.left;

  // Find the intersection point
  const intersectionPoint = getIntersectionPoint(cursorX);
  if (intersectionPoint) {
    // Move the intersection circle to the calculated point
    circleRef.current.setAttribute("cx", intersectionPoint.x);
    circleRef.current.setAttribute("cy", intersectionPoint.y);
    circleRef.current.style.display = "block";
  }
};

Finally, this would be the structure of our graph component

return (
  <svg ref={svgRef} height={height} width={width}>
    {drawPath()}
    {drawXAxis()}
    {drawYAxis()}
    {drawTrackingCircle()}
    {drawDataPointCircles()}
  </svg>
);

This is how we can use our Graph component

<LineGraph
  width={300}
  height={400}
  coordinates={samplePoints}
  lineColor="#000"
  pathColor="#00008B"
  areaColor="#ADD8E6"
  dataPointColor="#008000"
  showFillArea
  showDataPointCircle
/>

Codesandbox link for the LineGraph demo

Blog Photo by Isaac Smith on Unsplash

Thanks for reading ❤

Comments

[Akshay Gupta]

Just an extra point for people reading, we can also add event listener for mouseleave along with mousemove, which will hide the circle

  const handleMouseLeave = () => {
    if (circleRef?.current) {
      circleRef.current.style.display = 'none';
    }
  };