Electronics shop's sales – analyzing seasonality

We have data for sales in thousands of USD for a small electronics shop, by month, for the years 2010 to 2017. We would like to predict sales for each month of 2018:

 

Month/Year	2010	2011	2012	2013	2014	2015	2016	2017	2018
January	10.5	11.9	13.2	14.6	15.1	16.5	18.9	20
February	11.9	12.6	14.4	15.4	17.4	17.9	19.5	20.8
March	13.4	13.5	16.1	16.2	17.2	19.6	19.8	22.1
April	12.7	13.6	14.9	17.8	17.8	20.2	19.7	20.9
May	13.9	14.6	15.7	17.8	18.6	19.1	20.8	21.5
June	14	14.4	15.3	16.1	18.9	19.7	21.1	22.1
July	13.5	15.7	16.8	17.4	18.3	19.7	21	22.6
August	14.5	14	15.7	17	17.9	20.5	21	22.7
September	14.3	15.5	16.8	17.2	19.2	20.3	20.6	21.9
October	14.9	15.8	16.3	17.9	18.8	20.3	21.4	22.9
November	16.9	16.5	18.7	20.5	20.4	22.4	23.7	24
December	17.4	20.1	19.7	22.5	23	23.8	24.6	26.6

The data in the table is sales in thousands of USD

Analysis

To be able to analyze this, we will first graph the data so that we can observe patterns and analyze them:

From the graph and table, we notice that, in the long term, sales increase linearly. This is the trend. However, we can also see that the...