Get Y coordinate from the X coordinate in android Path

Question

The task seems simple: I need to draw a chart where every x coordinate has only one y coordinate (so it is a line). When it is drawn, a user can touch that chart and on the position he touched I should draw, let's say, a green circle. But it should be drawn on that chart:

Is it even possible? I tried to use PathMeasure , but all I can get is the Y coordinate for the distance, which is not what I want. If it's not possible with Path s, maybe there is another way of doing it?

Answer 1

It's a problem of geometry, finding the equation coefficients for a straight line passing through two points .

First you need to get the endpoints of the segment intersecting the vertical line containing the touch point (T[tx, ty]) namely: the point P[px, py] with the maximum x less than touched tx and the point Q[qx, qy] with the minimum x greater than tx

@NonNull
Point[] findSegment(@NonNull Point touched, Point[] dataPoints) {
    Point[] segment = new Point[2];
    if (dataPoints != null && dataPoints.length >= 2) {
        segment[0] = dataPoints[0];
        int i = 1;
        while (i < dataPoints.length && p.x < touched.x) {
            segment[0] = p;
            i++;
        }
        if (i < dataPoints.length - 1) {
            segment[1] = dataPoints[i];
        }
    }
    return segment;
}

//where Point is a class with two numeric fields
class Point {
    public double x; 
    public double y;
}

Then you need to compute the coefficients for the equation of the straight line (y = m * x + n) containing the segment using the two endpoints chosen previously:

The slope (gradient) is

m = (segment[0].y - segment[1].y) / (segment[0].x - segment[1].x);

The y intersect is

n = segment[0].y - m * segment[0].x // derived from y - y0 = m (x - x0) where x = 0 (at intersection with the vertical axis)

Then just compute y for the center of the circle using the equation replacing x by touched.x

   touched.y  = m * touched.x + n;

Take care to handle special cases: touched.x < firstPoint.x or touched.x > lastPoint.x (if that can occur) if you have vertical segments (trying to compute slope produces infinity or division by zero)