The Splunk PDF generation library has limitations on the width and height of the chart that it can produce. Current design is, when the height or the width of the chart cannot accommodate the ticks, they will be hidden.
However, there is a workaround for this behaviour. Instead of the "Export PDF" option, you could instead use the "Print" option, and in the menu, select the "Save as PDF" to generate the desired file. The PDF generated this way would look the same as the dashboard.
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