[ML] Anomaly marker on Single Metric Viewer chart not always on correct chart bucket

[peteharverson]

If the aggregation interval used for plotting data in the Single Metric Viewer chart is greater than the bucket span of the job, the marker indicating the anomaly may not always be positioned on the correct chart point.

Currently if there is not a chart point with the same time as the anomaly, the marker is placed on the chart point which is closest in time to the anomaly, which may be before or after the anomaly. This can lead to cases like the one shown below, where the marker does not seem to correspond to the anomaly in the data.

Here the job has a bucket span of 1 hour, whilst the chart data is plotted at 12 hour intervals. The time of the highlighted critical anomaly is from 10am to 11am on July 28th. The red marker is placed on the nearest chart point, at 1pm on July 28th, whereas the spike in the chart data is in the 1am point on July 28th:

If there is not a point in the chart which matches the time of the anomaly, the marker should be placed on the previous chart point which corresponds to the bucket containing the anomaly.

[elasticmachine]

Pinging @elastic/ml-ui