draw only sub-sample of huge datasets

This enables much faster drawing on datasets with more than width
graph pixels, but decreases graph resolution

diff --git a/public/facet_graph.js b/public/facet_graph.js
index 70ab79d..e20a4fc 100644 (file)
--- a/public/facet_graph.js
+++ b/public/facet_graph.js
@@ -75,7 +75,13 @@ ctx.fillStyle = '#ffcc88';
 ctx.moveTo( 0, 0 );
 ctx.beginPath();
 
-for( var i in data.x.data ) {
+var elements = data.x.data.length;
+var step = elements / data.width;
+console.debug( elements, step );
+if ( step < 1 ) step = 1;
+var i_float = 0;
+while( i_float < elements ) {
+       var i = i_float.toFixed();
        var x = data.x.data[i];
        if ( data.numeric ) x = Math.ceil( ( x - data.x.min ) / data.x.range * data.width  );
        var y = Math.ceil( ( data.y.data[i] - data.y.min ) / data.y.range * data.height );
@@ -88,6 +94,7 @@ for( var i in data.x.data ) {
                ctx.fillRect( x_px, 0, data.x.inc_bar, -y );
                ctx.strokeRect( x_px, 0, data.x.inc_bar, -y );
        }
+       i_float += step;
 }
 
 ctx.stroke();