Birth Year Histogram

Requirements

• Start idle and fetch GET /api/demo/birth-years only when Load birth years is clicked. Disable Load while pending. The response is {years:number[]}; all supplied demo data is fictional.
• Keep integer years from 1900 through 2029; ignore all other entries. Group valid years by decade (1980 means 1980–1989), sort ascending, and include zero-count decades between the first and last valid bucket.
• Implement histogram(years) with decade, count, and height (percentage of the maximum bucket count). Show total valid people.
• Author the chart CSS: equal-width decade columns with a 60px minimum width, a 160px plot row, aligned bottom baselines, proportional bar heights using --height, readable count/year rows and horizontal overflow for narrow screens. Do not use a chart library.
• Show pending, empty, HTTP/network-error and malformed-response states. A failed load can be retried and must not leave stale bars. Ignore late responses after unmount. Keep supplied IDs and accessible count labels.

Examples

Input: Load years [1982,1987,1991,1994,1999,2005]
Output: Show counts 2, 3 and 1 for the 1980s, 1990s and 2000s.

Input: Load [1980,2000]
Output: Include a zero-height 1990s bucket.