Mobility Rate Bubble Chart
This bubble chart shows the relationship between median parent household income and mobility rate for California universities, with circle size encoding average cohort size and color encoding income trend changes. The visualization uses D3.js v5 with d3.legend to create both color and size legends, plus custom axis and title drawing functions. Data is loaded from an Opportunity Insights CSV and filtered to California institutions, with pre-selected universities labeled on the chart. SVG rendering displays the bubbles, median reference lines, and labels.