(I say “have a tendency to rise” and “have a tendency to fall” because of course there are many other things which have an influence on prices, including changes in international prices, changes in government charging policy, changes in tariff policies, and similar factors.)
The slope of the line, in other words the trend rate of growth in the economy’s capacity to supply, is largely driven by factors like the growth in the labour force, the quality of the education system, the quality of management decision-making, the openness of the economy, the quality of the judicial system, the quality of the industrial relations framework, and so on.