Legible Label Layout for Data Visualization, Algorithm and Integration into Vega-Lite

TL;DR

This paper introduces a bitmap-based label placement algorithm that efficiently prevents label overlaps in large, complex data visualizations, significantly improving performance over previous methods and integrating into Vega-Lite.

Contribution

We present a novel bitmap-based algorithm for label placement that enhances efficiency and scalability, and integrate it into Vega-Lite as a label encoding channel.

Findings

Significant performance improvements over existing algorithms.

Effective placement of labels in charts with many complex marks.

Successful integration into Vega-Lite for practical use.

Abstract

Legible labels should not overlap with other labels and other marks in a chart. When a chart contains a large number of data points, manually positioning these labels for each data point in the chart is a tedious task. A labeling algorithm is necessary to automatically layout the labels for a chart with a large number of data points. The state-of-the-art labeling algorithm detects overlaps using a set of points to approximate each mark's shape. This approach is inefficient for large marks or many marks as it requires too many points to detect overlaps. In response, we present a bitmap-based label placement algorithm, which leverages an occupancy bitmap to accelerate overlap detection. To create an occupancy bitmap, we rasterize marks onto a bitmap based on the area they occupy in the chart. With the bitmap, we can efficiently place labels without overlapping existing marks, regardless of the number and geometric complexity of the marks. This bitmap-based algorithm offers significant performance improvements over the state-of-the-art approach while placing a similar number of labels. We also integrate this algorithm into Vega-Lite as one of its encoding channels, label encoding. Label encoding allows users to encode fields in each data point with a text label to annotate the mark that represents the data point in a chart.