Fix hassgraph crash on empty or non-numeric HA states.

[tavdog]

Skip invalid history states before float conversion so raw data plotting and stats no longer fail when Home Assistant returns empty strings.

[gemini-code-assist]

Code Review

This pull request introduces a parse_state_value helper function to safely parse Home Assistant state values and refactors the codebase to use it. Feedback identifies a potential runtime crash if state is None and suggests preserving the original string instead of defaulting to 0 when rounding non-numeric states.

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[gemini-code-assist]

Converting a non-numeric state (like "unavailable") to 0 when rounding is enabled is misleading. Skip rounding and preserve the original string if parse_state_value returns None.

Suggested change

	val = parse_state_value(current_value)
	if val == None:
	val = 0
	val = round(val, decimal)
	current_value = str(int(val)) if val == int(val) else str(val)
	val = parse_state_value(current_value)
	if val != None:
	val = round(val, decimal)
	current_value = str(int(val)) if val == int(val) else str(val)