Please, check the code snippet below to see how you can achieve the desired effect. The PadRight() method aligns to the left the string by adding the necessary number of spaces until the the total number of characters is 15. If the initial string is, for example, 5 characters long, this method will add 10 spaces on the right of the string. As you already understand, the PadLeft() method does the same but in the other direction. I hope this will help you to achieve the desired result.
ChartSeriesItem chartItem = item.ChartItem;
chartItem.Label.TextBlock.Text = str1.PadRight(15) + str2.PadLeft(15);
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