Diagonal Alphabet To the Input

Charcoal, 9 bytes

↘✂β⁰Ｎ↓✂βη

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How it works

↘✂β⁰Ｎ↓✂βη
 ✂β⁰Ｎ         the alphabet from position 0 to the input
↘               print diagonally, down and to the right
        ✂βη    the alphabet starting from the position of the input
       ↓        print downwards

This solution no longer works in the current version of Charcoal (most likely due to a bug fix), but the issue is resolved for 10 bytes with ↘✂β⁰Ｎ↓✂βＩθ.

05AB1E, 11 bytes

AvNI<‚Wysú,

First time trying 05AB1E, so I'm open to tips.

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If a zero-indexed input from 0 to 25 is allowed, this can be 10 bytes by omitting the <.

Python 2, 61 58 57 56 bytes

def f(i,j=0):exec"print(' '*~-i)[:j]+chr(97+j);j+=1;"*26

-3 bytes thanks to Rod

-2 more bytes thanks to Mr. Xcoder