When all y values in a data set are 0 the y-axis range is -1 to 1. I tried overriding the ymin value, but the change did not take effect because the ymin == ymax.
Morris.Line({
element: 'sales_chart',
data: data['product_analytics'],
xkey: 'time',
ykeys: ['sales'],
ymin: 0,
labels: ['Total Sales'],
smooth: false,
preUnits: '$'
});