A business selling plumbing equipment to the trade (i.e. professional plumbers only) increases the price of copper piping by 4 per cent and reduces the price of radiators by 5 per cent. A year later the company analyses its sales figures and finds that revenue for copper piping rose in the first three months after the price rise but then fell dramatically thereafter, while the revenue for sales of radiators also fell throughout the period. Explain what might have happened to bring about this situation. Illustrate your answer with diagrams where appropriate.