I don't understand what you mean by "join the points"...
maybe you plotted points and then you joined them to draw the complete parabola.
the parabola is a continuous function and just because you have plotted points from an equation does not mean that is representative of this function. In order to represent the function well graphically you must connect the points on the graph.
you plotted x=0, x=1, x=5
But what happens when x=3, drawing the line gives you an idea of what happens between the points.