This document discusses the experience of data and materiality through indexical design. It explores how images and visualizations can help build knowledge by not just representing data symbolically, but also showing causal relationships and physical evidence through indexical traces. Examples are given of data visualizations that function indexically by framing phenomena, ambient displays that can be read like traces, and citizen science projects that establish physical evidence. The document examines how traces connect representational practices to the physical world and are discovered through the act of reading them, sitting at the intersection of meaning and materiality.