If x' and x" be the x-coordinate of the particle at initial time t' and t" respectively then vx =(x"-x')/(t"-t') = tanθ.
For t"-t' infinitesimally small it is the vx at that instant.
So slope of the tangent at any point in the above graph gives vx .
At t=t1, tanθ is positive, so sign of vx is positive.
At t= t2 the slope of the curve is horizontal, so tanθ=0 → vx =0.
At t=t3 the slope of the curve is negative, so sign of vx is negative.