Ionogram is chart X axle is Frequency , Y axle is height of reflection ( ionosphere height )
I understand this chart is reflection from ionosphere sending radio wave vertical up. so if sending angle is not vertical reflection does extend more to higher frequency. But, it is good reference for Ham to see if band is open or not.
LINK to Point Arguello , CA, USA data http://car.uml.edu/common/DIDBYearListForStation?ursiCode=PA836
there are many place which takes this data through out the world
click this link for lists http://car.uml.edu/common/DIDBFastStationList
click this link for world wide map http://ulcar.uml.edu/stationmap.html
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