Given a list of conversion items between symbols, write an algorithm to return the conversion ratio for each pair of symbols.

This problem can be solved using a graph-based approach. Represent the conversion items as a directed graph where symbols are nodes and conversion rates are edge weights. To find the conversion ratio between two symbols, perform a Breadth-First Search (BFS) or Depth-First Search (DFS) starting from the source symbol. During the traversal, keep track of the cumulative conversion ratio. If the destination symbol is reached, return the accumulated ratio. If the destination is unreachable or cycles are encountered without reaching the destination, return an indicator of failure (e.g., -1.0 or null). Ensure to handle cycles appropriately to avoid infinite loops.