Percentage change from the previous line when multiple lines have the same date?

I have a query that calculates the relationship and returns it for every hour and server on a specific day:

SELECT a.day, 
       a.hour, 
       Sum(a.gemspurchased), 
       Sum(b.gems), 
       Sum(b.shadowgems), 
       ( Sum(b.gems) / Sum(a.gemspurchased) )       AS GemRatio, 
       ( Sum(b.shadowgems) / Sum(a.gemspurchased) ) AS ShadowGemRatio 
FROM   (SELECT Date(Date_sub(createddate, INTERVAL 7 hour)) AS day, 
               Hour(Date_sub(createddate, INTERVAL 7 hour)) AS hour, 
               serverid, 
               Sum(gems)                                    AS GemsPurchased 
        FROM   dollartransactions 
        WHERE  Date(Date_sub(createddate, INTERVAL 7 hour)) BETWEEN 
               Curdate() - INTERVAL 14 day AND Curdate() 
        GROUP  BY 1, 
                  2, 
                  3) a, 
       /*Gems recorded from DollarTransactions Table after purchasing gem package*/ 
       (SELECT Date(Date_sub(createddate, INTERVAL 7 hour)) AS day, 
               Hour(Date_sub(createddate, INTERVAL 7 hour)) AS hour, 
               serverid, 
               Sum(acceptedamount)                          AS Gems, 
               Sum(acceptedshadowamount)                    AS ShadowGems 
        FROM   gemtransactions 
        WHERE  Date(Date_sub(createddate, INTERVAL 7 hour)) BETWEEN 
                       Curdate() - INTERVAL 14 day AND Curdate() 
               AND transactiontype IN ( 990, 2 ) 
               AND fullfilled = 1 
               AND gemtransactionid >= 130000000 
        GROUP  BY 1, 
                  2, 
                  3) b 
/*Gems & Shadow Gems spent, recorded from GemTransactions Table */ 
WHERE  a.day = b.day 
       AND a.serverid = b.serverid 
GROUP  BY 1, 
          2 

      

        
        
        
      

    

This code returns the constituent parts of the relationship, as well as the relationship itself (which is sometimes null):

day       hour  sum(a.GemsPurchased)    sum(b.Gems) sum(b.ShadowGems)   GemRatio ShadowGemRatio
9/5/2014    0   472875                   465499      60766              0.9844    0.1285
9/5/2014    1   350960                   371092      45408              1.0574    0.1294
9/5/2014    2   472985                   509618      58329              1.0775    0.1233
9/5/2014    3   1023905                  629310      71017              0.6146    0.0694
9/5/2014    4   1273170                  628697      74896              0.4938    0.0588
9/5/2014    5   998920                   637709      64145              0.6384    0.0642
9/5/2014    6   876470                   651451      68977              0.7433    0.0787
9/5/2014    7   669100                   667217      81599              0.9972    0.122

      

        
        
        
      

    

What I would like to do is create 8th and 9th columns that calculate the% change from the previous row for both GemRatio and ShadowGemRatio. I've seen other threads here on how to do this for specific queries, but I couldn't get it to work for my specific MySQL query ...