Human eye cannot process effectively more than a few (thousands) datapoints per View.
Additionally, in Data Visualization you have other restrictions:
- number of pixels on your screen (may be 2-3 millions maximum) available for your View (Chart or Dashboard).
- time to render millions of Datapoints can be too long and may create a bad User Experience (too much waiting).
- time to load your Datapoints into your View; if you wish to have a good User Experience, than 2-3 seconds is maximum user can wait for. If you have a live connection to datasource, than 2-3 seconds mean a few thousands of Datapoints maximum.
- again, more Datapoints you will put in your View, more crowded it will be and less useful and less understandable your View will be for your users.
Recently, some Vendors started to add new reason for you (called Local Rendering) to restrict yourself in terms of how much of Datapoints you need to put into your DataView: usage of Client-side hardware (especially its Graphical Hardware) for so called “Local Rendering”.
Local rendering means that Data Visualization Server will send DataPoints instead of Images to Client and Rendering of Image will happened on Client-side, using capability of modern Web Browsers (to use Client’s Hardware) and HTML5 Canvas technology.
For example, the new feature in Tableau Server 8 will automatically switch to Local Rendering if number of DataPoints in your DataView (Worksheet with your Chart or Dashboard) is less then 5000 DataPoints (Marks in Tableau Speak). In addition to faster rendering it means less round-trips to Server (for example when you hover your mouse over Datapoint, in old world it means round-trip to Server) and faster Drill-down, Selection and Filtering operations.
Update 3/19/13: James Baker from Tableau Software explains why Tableau 8 Dashboards in Web Browser feel more responsive:
James explained that “HTML5’s canvas element” is used as drawing surface. He underscored that it’s much faster to send images rather than data because image size does not scale up linearly. James included a short video shows incremental filtering in a browser, one of the features of Local Rendering.