traceCouplingsWith = cellTraceFunc => painter.trace(trace => {
            for (let row = 0; row < numRows; row++) {
                for (let col = 0; col < numCols; col++) {
                    let k = (row * numCols + col) * 2;
                    cellTraceFunc(
                        trace,
                        buf[k],
                        buf[k + 1],
                        x + diam * col,
                        y + diam * row,
                        diam);
                }
            }
        })
@param {!Painter} painter
@param {!Matrix} matrix
@param {!Rect} drawArea
@param {!Array.<!Point>} focusPoints
@param {!string=} backgroundColor
@param {!string=} fillColor