How can I create a fake datapoint in Chart.js line chart for the current day label?

To plot the value of blueline for the label "2023-03-26", you first need to find the index of "2023-03-26" in the datelabel array, and then use that index to find the corresponding value in the blueline array.

const targetDate = "2023-03-26";
const index = datelabel.indexOf(targetDate);

To calculate the value for a missing date, you can interpolate between the two nearest known values. In this case, since you only have the first and last values in the blueline array, you can perform a linear interpolation between these two values based on the index of your target date.

Here is how you can do it:

const firstKnownIndex = blueline.findIndex(value => !isNaN(value) && value !== null);
const lastKnownIndex = blueline.lastIndexOf(blueline.find(value => !isNaN(value) && value !== null));

const firstKnownValue = blueline[firstKnownIndex];
const lastKnownValue = blueline[lastKnownIndex];

const index = datelabel.indexOf(targetDate);

if (index !== -1 && firstKnownIndex !== -1 && lastKnownIndex !== -1) {
    const interpolatedValue = firstKnownValue + ((index - firstKnownIndex) * (lastKnownValue - firstKnownValue)) / (lastKnownIndex - firstKnownIndex);
    console.log(`The interpolated value of blueline for the date ${targetDate} is:`, interpolatedValue);
} else {
    console.log(`The date ${targetDate} was not found in the datelabel array, or there are no known values to interpolate between.`);
}

Add then the value to blueline in the correspondent index and you will see the desired point in the line chart.