Add y lines option for grid

c3.js

@@ -2260,7 +2260,7 @@
             // Y-Grid
             if (withY && __grid_y_show) {
                 ygrid = main.select('.ygrids').selectAll(".ygrid")
-                    .data(y.ticks(10));
+                    .data(y.ticks(__grid_y_lines));
                 ygrid.enter().append('line')
                     .attr('class', 'ygrid');
                 ygrid.attr("x1", __axis_rotated ? y : 0)