"triangular" sliders when plotting with @interact ?

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Hello! I am interested in plotting a function $f_{u}(x)$ (or $f_u(x,y)$) using @interact. The subscript $u = (u_1, u_2, u_3)$ is a vector in the $2$-dimensional simplex, so $u$ is a real-valued vector with nonnegative entries summing to $1$.

I am aware of single-variable sliders. For the plot of this function $f_u$, it seems natural to me to adjust by clicking a point in this simplex, where the simplex is drawn off to the side as an equilateral triangle with corners labeled $(1,0,0), (0,1,0), (0,0,1)$.

Are features like this built-in? I am okay to use either notebook!