Thank you for the attachments and explanations.
Our chart offers a feature called Sampling. To improve performance and to avoid unclear visual representation when dealing with a huge number of items (in the ItemsSource) the chart combines (clusters) some data items into a single data point. For example - If you have 4000 data items in your items source - only 200 data points will be visualized (200 is the default, but you can control this number). This means that if you used bar series - only 200 bars will be shown, each bar corresponding to one data point, and this data point contains the information for 20 data items. The values for the X and Y in this data point are somehow summarized (by default it is the Average, but you can control this too).
Now, in the snapshot you have attached - the chart on the left shows information for 4000 data items, but it only visualizes 400 data points (you said you have set it to 400). Each data point holds average information for 10 data items. This means that if the X values in these data items range from the 13th day to the 23rd day of the same month and year - an average estimate will be held in the data point - the 18th day of that month and year.
And this is why when not zoomed in the tooltip shows 13 and when you zoom in it shows the actual 19. In other words - data is not missing from the chart but it is "averaged".
I am not familiar with your scenario and I can't make any suggestions for you but here are a couple of things that you can consider.
1. Simply not show a date in the tool tip.
2. Create a custom sampling function. Custom Sampling Functions can be very helpful in situations where you do not want to disable the Sampling feature (I have left a link below)
3. Fully disable sampling. This way no clustering will be done and no average values will be passed and shown - only the actual data. But keep in mind that this will slow performance and might make things a bit unclear.
Please read about Data Sampling ad Custom Sampling Functions here
You might also be interested in using format expressions
for your tooltips, as many things can be achieved in XAML.
I hope this helps.
the Telerik team
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