According to the diagram, there is no corresponding "straight through" lane on the other side of the intersection where you were stopped.
If this is an accurate depiction, then the left turn sign may have been painted on the pavement and you didn't see it or it has faded to the point where it was difficult to discern.
Continuing, if your diagram is accurate, then it would seem reasonable to a reasonable and experienced driver that this was indeed a left turn lane.
According to your description, you used your turn signal to show you did not want to go left but merge into the next lane to your left. Why did you do that if there was a lane for you to proceed straight on?
Was the traffic signal a full green light or just a green straight arrow (which would indicate that there would be a green left arrow light on the same signal)?
If the former, you may be OK. If the latter, you may have a violation.
According to the diagram, there is no corresponding "straight through" lane on the other side of the intersection where you were stopped.
If this is an accurate depiction, then the left turn sign may have been painted on the pavement and you didn't see it or it has faded to the point where it was difficult to discern.
Continuing, if your diagram is accurate, then it would seem reasonable to a reasonable and experienced driver that this was indeed a left turn lane.
According to your description, you used your turn signal to show you did not want to go left but merge into the next lane to your left. Why did you do that if there was a lane for you to proceed straight on?
Was the traffic signal a full green light or just a green straight arrow (which would indicate that there would be a green left arrow light on the same signal)?
If the former, you may be OK. If the latter, you may have a violation.