It's not possible to read the Y property of the context element from a Chart.js BoxAnnotation

I'm new to using Chart.js and I've done a lot of research, but haven't found anything that could help me, so I'm reaching out to the community for assistance.

I'm working on a project using Chart.js version 3.2.1 and chartjs-plugin-annotation version 1.0.1. Unfortunately, updating the libraries is not possible at the moment.

Here's the scenario: I need to fill the background of a BoxAnnotation plugin with a gradient. Okay, I've figured out how to do that. The challenge is that this BoxAnnotation varies in location based on the user-applied filter. Therefore, the Y coordinate of the createLinearGradient needs to be obtained from the BoxAnnotation at runtime.

Now the problem is that I can't read the Y property of the context element. It always returns undefined. Consequently, I can't correctly set where the gradient should start.

Here's the code for the BoxAnnotation:

boxDeficitStatus: !this.onlyVirtualSensors && {
    type: 'box',
    yScaleID: 'right-y-axis',
    drawTime: 'beforeDatasetsDraw',
    yMin: 0,
    yMax: $this.moistureCritical,
    backgroundColor: function(context) {
        const chart = context.chart;
        const {ctx, chartArea} = chart;

        return getGradient(ctx, chartArea);
    },
    borderWidth: 0,
    display: !this.onlyVirtualSensors,
}

And here's the code for the getGradient function:

function getGradient(ctx, chartArea) {
  const chartHeight = chartArea.bottom - chartArea.top;

  //Instead of 246, the Y coordinate value I'm trying to obtain should go there.
  const gradient = ctx.createLinearGradient(67.7, 246, 67.7, chartHeight);
  gradient.addColorStop(0, "rgba(255, 255, 255, 0.3)");
  gradient.addColorStop(1, "rgba(255, 0, 0, 0.3)");
  
  return gradient;
}

When running a console.log('$context', context.element['$context'])

The result in the browser terminal is as shown in the image you provided:

However, when trying to access the Y property using any of the following:

console.log('y', context.element['$context']['element'].y)
console.log('y', context.element.y)
console.log('y', context.element['y'])

The result is always undefined.

My question then is whether it's possible to access this Y property somehow or if there's another way to obtain the coordinate I need.

You're welcome! I appreciate your patience in advance!

I managed to overcome this challenge, and I'll share the solution for anyone who might need it in the future.

The issue was that the values of the 'element' in the context were calculated right after the chart was initialized. Therefore, I needed to find a way to access the values I needed immediately after the chart rendering. To achieve this, I used setTimeout and chart.update, as our friend @kikon had suggested.

The solution ended up being essentially as follows:

import { Component, OnInit } from '@angular/core';
import { Chart } from 'chart.js';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent implements OnInit {
  myChart: Chart;
  chartCtx: any;
  elementCtx: any;

  ngOnInit() {
    this.plotChart();
    this.getStatusBoxCoordinates(this.elementCtx);
  }

  getStatusBoxCoordinates(element) {
    setTimeout(() => {
      const boxStatus=
        this.myChart.options.plugins.annotation.annotations[
          'boxStatus'
        ];
      boxStatus.backgroundColor = this.getGradient(
        element?.x,
        element?.y,
        this.chartCtx
      );
      this.myChart.update();
    }, 50);
  }

  getGradient(coordinateX, coordinateY, chart) {
    const { ctx, chartArea } = chart;
    const x = coordinateX || 0;
    const y0 = coordinateY || 0;
    const y1 = chartArea.bottom - 50 || 295;
    const gradient = ctx.createLinearGradient(x, y0, x, y1);
    gradient.addColorStop(0, 'rgba(255, 229, 229,0.1)');
    gradient.addColorStop(0.2, 'rgba(255, 198, 198,0.1)');
    gradient.addColorStop(0.4, 'rgba(252, 175, 175,.1)');
    gradient.addColorStop(0.6, 'rgba(249, 130, 130,0.1)');
    gradient.addColorStop(0.8, 'rgba(242, 109, 109, .1)');
    gradient.addColorStop(1, 'rgba(224, 76, 76, 0.1)');

    return gradient;
  }

  plotChart() {
    let $this = this;

    const xValues = [50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150];
    const yValues = [7, 8, 8, 9, 9, 9, 10, 11, 14, 14, 15];

    this.myChart = new Chart('myChart', {
      type: 'line',
      data: {
        labels: xValues,
        datasets: [
          {
            fill: false,
            backgroundColor: 'rgba(0,0,255,1.0)',
            borderColor: 'rgba(0,0,255,0.1)',
            data: yValues,
          },
        ],
      },
      options: {
        plugins: {
          annotation: {
            annotations: {
              boxStatus: {
                type: 'box',
                yScaleID: 'y',
                yMin: 0,
                yMax: 10,
                backgroundColor: function (context) {
                  $this.elementCtx = context.element;
                  $this.chartCtx = context.chart;
                  return 'rgba(255, 255, 255, 0)';
                },
              },
            },
          },
        },
      },
    });
  }
}

I utilized variables accessible throughout the component to store the context of boxStatus, and then I updated the backgroundColor based on this context after the chart had already calculated y0.

Well, that's it! I hope this helps someone in the future!