Give them as one big array. If you have them in multiple arrays you can use the JavaScript concat() function or you just add them together like below.
Here's an example without using the concat function:
new RGraph.Line({
id: 'cvs',
data: [
[8,4,3],
[9,8,1]
],
options: {
tooltips: ['A', 'B','C','D','E','F']
}
}).draw();
And here's an example using concat():
tooltips_a = ['A', 'B','C'];
tooltips_b = ['D','E','F'];
tooltips_combo = tooltips_a.concat(tooltips_b);
new RGraph.Line({
id: 'cvs',
data: [
[8,4,3],
[9,8,1]
],
options: {
tooltips: tooltips_combo
}
}).draw();