The arrayQualityMetrics package, authored by Audrey Kauffmann and Wolfgang Huber, can produce a report on microarray quality metrics. Within these reports is a scatterplot of principal components. The plot features interactivity by way of increasing the size of a point once it is hovered over. arrayQualityMetrics produces this plot through the use of the SVGAnnotation package.
An attempt has been made by Paul Murrell to replicate this type of plot (including interactivity) using gridSVG.
By hovering over each of the points, we are able to highlight the point by enlarging it. Not only is the point enlarged, but the name of the point is shown on the bottom-left of the plot.