Plotalot uses the Google Charts API, which is a library of Javascript functions. The library renders charts using HTML5 Scalable Vector Graphics (SVG). Vector Markup Language (VML) is used for older versions of Internet Explorer. The library works with all modern browsers including those on iPhones, iPads and Android. Charts are rendered on the client computer, using the browser's Javascript engine. Google's servers are not involved in rendering the charts, and the chart data is not sent to Google or any other server. Google simply serves the Google Charts Javascript library.
Plotalot retrieves chart data from the database by executing the SQL select statement(s) configured for the chart. It then analyses the data to determine the data types and value ranges, and builds the Javascript data structure in the correct format for Google Charts API and the specific chart. For charts with multiple plots, the datasets must be merged into a single dataset, and in some cases re-sorted. Values and strings are formatted, encoded and escaped as required. Plotalot then produces the rest of the Javascript needed to call the Charts API, based on the options selected. The Javascript is added to the Joomla page being generated, and is executed by the browser on the client computer, with the chart eventually being rendered in the chosen <div> element.
Part of Plotalot's analysis decides on the number of data points to include in the Javascript. There is no point sending more data points to the browser than the chart has pixels. Doing so would be unnecessarily slow, and might even crash the browser. For large datasets, Plotalot samples the data to provide accurate charts without creating unduly large scripts. The sampling process does mean that data "spikes" could be missing from the data sent to the browser, but there is really no choice, and it's likely that spikes of less than a pixel would not be visible in any case.
You can examine the Javascript produced by Plotalot by looking in the Script tab in the chart area, or by viewing the source of a web page containing Plotalot charts.