The temperature is staying the same. In the graph when it shows solid/liquid and liquid/gas, the temperature stays the same until it changes. This is because it reached it's melting point/vaporizing point. For example, a solid gets heated up, it then reaches it's melting point but it can't go higher than that because it isn't fully a liquid yet, once it's a liquid it will then continue to rise in temperature.
I don't think I put enough detail into that explanation but I hope this helps your problem.
