Tensorflow summary: adding a variable which does not belong to computational graph

Yes, you can create summaries outside the graph.

Here is an example where the summary is created outside the graph (not as a TF op):

output_path = "/tmp/myTest"
summary_writer = tf.summary.FileWriter(output_path)

for x in range(100):
   myVar = 2*x

   summary=tf.Summary()
   summary.value.add(tag='myVar', simple_value = myVar)
   summary_writer.add_summary(summary, x)

summary_writer.flush()