Google Charts - Curved Line Chart incorrectly dips below 0
The curve may continue to dip below 0 no matter what you do, but you can crop the view so the lowest point of the displayed graph is at 0. You can do that with the vAxis.viewWindow.min property:
lineChart.draw(data,
{
curveType: "function",
vAxis: {viewWindow: {min:0} }
}
);
See the LineChart documentation for information on vAxis.viewWindow.min and other configuration options.
Just a quick update on this. I realised that my data work not be suitable for a curved graph anyway, as it is discrete data and not continuous. I had to switch back to straight lines, which removed my problem. Not an ideal solution, I know, but it is one that worked for me.