VOPlot is a Java applet. This means you have to have a java virtual machine active within your browser. A common way to get this is to install a plugin, available, e.g., from java.sun.com, or by installing a package like openjdk-?-jre on Debian-derived systems (where ? corresponds to some version).

Also, this HTML page assumes you run a post-Y2k (i.e., at least HTML 4.01) standards-compliant browser. There are plenty of those out there.

If neither applet nor fallback content appears above, your browser's java support is broken. Sometimes a reload helps.

VOPlot is developed by Persistent Systems in cooperation with VO india. Standalone versions and user guides are available at VO India's VOPlot page.