How to generate a grid mesh

Question

Hi!
I'm trying to make a dynamic ocean mesh but I can't figure out how to render the mesh correctly

In short, I'd just like to render a 10x10 mesh grid that looks like a plane
How can I do that ?

Here's how it looks like in my head aha

Here is what I got so far (I can see weird triangles):

extends Spatial

onready var ig:ImmediateGeometry = $ImmediateGeometry
var t:float = 0.0

func _ready():
    pass

func _process(delta):
    ig.clear()
    ig.begin(Mesh.PRIMITIVE_TRIANGLE_STRIP)
    for i in range(-2, 2):
        for j in range(-2, 2):
            var height = sin(t + i + j)
            ig.add_vertex(Vector3(j, height, i))
    ig.end()

    t += delta
    if t >= 2 * PI:
        t = 0.0