barycentric interpolation

see https://observablehq.com/@visionscarto/igrf-90

Author: Fil

src/marks/raster.js

@@ -214,6 +214,6 @@ function interpolatePixel(index, canvas, {X, Y, R, G, B, FO}, {r, g, b, a}) {
       imageData[j + 2] = b;
     }
     imageData[j + 3] = FO ? FO[i] * 255 : a;
-    context2d.putImageData(image, 0, 0);
   }
+  context2d.putImageData(image, 0, 0);
 }

test/data/README.md

@@ -26,6 +26,10 @@ https://observablehq.com/@bmschmidt/data-driven-projections-darwins-world
 Bureau of Labor Statistics
 https://www.bls.gov/
 
+## ca55-south.csv
+Great Britain aeromagnetic survey
+https://www.bgs.ac.uk/datasets/gb-aeromagnetic-survey/
+
 ## cars.csv
 1983 ASA Data Exposition
 http://lib.stat.cmu.edu/datasets/