要绘制标签“2023-03-26”的blueline值，首先需要在datelab数组中找到“2023-03-26”的索引，然后使用该索引在blueline数组中找到相应的值。

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

要计算缺失日期的值，可以在两个最近的已知值之间进行插值。在这种情况下，由于blueline数组中只有第一个和最后一个值，因此可以根据目标日期的索引在这两个值之间执行线性插值。
以下是您可以做到的方法：

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.`);
}

然后将该值添加到相应索引中的蓝线，您将在折线图中看到所需的点。