From the raw data, calculate the centred moving average series and plot this on the graph.

\begin{tabular}{|c|c|c|} \hline Year & Period & Sales (000 s) \\ \hline 2017 & 1 & 30 \\ & 2 & 35 \\ & 3 & 35 \\ & 4 & 40 \\ & 5 & 50 \\ & 6 & 60 \\ \hline \end{tabular} \begin{tabular}{|c|c|c|} \hline Year & Period & Sales (000s) \\ \hline 2018 & 1 & 30 \\ & 2 & 40 \\ & 3 & 38 \\ & 4 & 35 \\ & 5 & 52 \\ & 6 & 60 \\ \hline \end{tabular} \begin{tabular}{|c|c|c|} \hline Year & Period & Sales (000s) \\ \hline 2019 & 1 & 35 \\ & 2 & 33 \\ & 3 & 37 \\ & 4 & 43 \\ & 5 & 50 \\ & 6 & 65 \\ \hline \end{tabular}